UNITED  STATES  DEPARTMENT  OF  AGRICULTURE 

BULLETIN  No.  1059 


Washington,  D.  C. 


Contribution  from  the  Forest  Service 
WILLIAM  B.  GREELEY,  Forester 


May  19, 1922 


RESEARCH  METHODS  IN  THE  STUDY 
OF  FOREST  ENVIRONMENT 

By 

CARLOS  G.  BATES,  Silviculturist 
In  Charge  Fremont  Forest  Experiment  Station 

and 

RAPHAEL  ZON,  Forest  Economist 


CONTENTS 


Page 

Introduction 2 

Measurement   of  Environmental    Condi- 
tions Affecting  Vegetation 11 

Climatic  Characteristics  of  Locality    ...  11 

Natural  Climatic  Regions 11 

Data  Obtained  by  the  Weather  Bureau .  1 1 
Knowledge  of  Existing  Stations  Nec- 
essary      12 

Periods  of  Growth  and  Re3t 12 

Special  Observations  on  Climate  and  Soil 

of  Locality 13 

Location  of  Instruments  for  Study  of 

Growth 13 

Location  of  Instruments  for  Study  of 

Reproduction 13 

Air  Temperatures 15 


Pap 


Special  Observations  on  Climate  and  Soil 
of  Locality — Continued. 

Soil  Temperatures .    .    .     26 

Solar  Radiation— Light 39 

Precipitation 60 

Soil  Moisture  and  Soil  Qualities ...     66 

Atmospheric  Humidity 143 

Wind  Movement 146 

Evaporation 151 

Phenology 168 

External  Field  Observations    .    .    .    .170 
Internal  or  Physiological  Observations.    1 7 1 

Field     Observations,     Photographs,     and 
Maps 172 

Appendix 175 

List  of  References 201 


Revised,  August,  1922 


jfr&'<$mru 


ASHINGTON 
T   PRINTING   OFFICE 
1922 


-*w 


LIBRA 

|!|  || 

in 

:s 

79   Y 

UNITED  STATES  DEPARTMENT  OF  AGRICl 

-  BULLETIN  No.  1059 


jfVJ^^Wl. 


Contribution  from  the  Forest  Service 
WILLIAM  B.  GREELEY,  Forester 


Washington,  D.  C. 


May  19,  1922 


RESEARCH  METHODS  IN  THE  STUDY   OF  FOREST 

ENVIRONMENT.1 

By  Carlos  G.  Bates,  Silviculturist  in  Charge  of  Fremont  Forest  Experiment  Station, 

and  Raphael  Zon,  Forest  Economist. 


CONTENTS. 


Page. 


11 

11 
12 


Introduction 

Object 

Scope 

Sample  plot  method 

Need  for  a  permanent  organization  in 
forest  investigations 

Forest  experiment  stations 

Short-term  studies. . : 

The  simple  physico-physiological  concept . 
.Icasurement    of    environmental    conditions 

affecting  vegetation H 

Climatic  characteristics  of  locality 

Natural  climatic  regions 

Data  obtained  by  the  Weather  Bureau. . 

Knowledge  of  existing  stations  necessary 

Periods  of  growth  and  rest 12 

Special  observations  on  climate  and  soil  of 

locality 13 

Location   of  instruments   for   study   of 

growth 13 

Location    of   instruments   for   study   of 

reproduction *3 

Air  temperatures 15 

Problems 

Exposure  of  thermometers 

Standardizing  thermometers 18 

Maximum,    minimum,    and    current 
temperatures 

Hourly  temperatures 19 

•     Frosts 24 

Mean  temperatures 24 

Annual  summary 

Instruments 

Soil  temperatures 

Purposes  to  be  served 

Problems 

Time  of  observations 28 

Daily  mean  soil  temperatures 

»  Revised,  August,  1922. 

10163— 22— Bull.  1059 1 


Page. 


Measurement  of  environmental  conditions  af- 
fecting vegetation— Continued. 
Special   observations   on   climate    and    soil   of 
locality —Con  tinued . 


NATURAL  RESOURCES 

THIS  BOOJOJS  M$¥)N  THE  DA^ 
INDICATE!)  BELOW  AND  IS  SU 
JECT  TO  AN  OVERDUE  FINE  i 
POSTED  AT  THE  CIRCULATK 
DESK.  , 

4/2  <W 


EXCEPTION:     C 

earlier  if  this  it* 


ate  du?  will  be 
mis  RECALLED 


BULLETIN   1059,   U.    S.    DEPARTMENT    OF   AGRICULTURE. 


Page. 
Measurement  of  environmental  conditions  af- 
fecting vegetation— Cunt  inued. 

Special    observations    on    climate    and    soil  of 
locality— Continued . 

Precipitation 6u 

Exposure  of  gauges 60 

Snow  depths 61 

Sn<>w-scalc  readings 61 

Tabulat  ion 63 

Instruments 63 

Soil  moist  ure  and  soil  qualities 66 

Osmosisasa  factor  in  water  absorption.  66 

Problems  and  some  definitions ..... . . .  71 

Total  moisture  determinations 73 

Soil  wells  for  representative  points. ..  73 

Technique  of  periodic  sampling 76 

Determination  of  nonavailable  moist- 
ure   79 

Direct  determination  of  wilting  coeffi- 
cient    SO 

Indirect  methods  for  wilting  coeffi- 
cient    S4 

Capillary  moisture 85 

Moisture  equivalent 93 

Hygroscopic  coefficient 97 

Calculation  of  the  available  moisture. .  100 

Availability  of  the  moisture 101 

Coefficient  of  availability 102 

Osmotic  pressure  in  plant  tissues. .  103 
Method  of  determining  freezing 

points 107 

Osmotic  pressure  in  soils 109 

Vapor  transfer  in  soils 109 

Vapor  transfer  method 112 

Computing  the  coefficient 120 

Other  soil  properties  to  be  studied 121 

Acidity  and  alkalinity 121 

Hydrogen-ion  concentration 122 

Mechanical  analysis  of  soils 123 

Determination  of  humus 123 

Loss  on  ignition 127 

Ammonia-soluble  humus 127 

Capillary  conductivity 128 

Chemical  analysis  for  nutrients 129 

Summary  of  soils  discussion 136 

Special  equipment 142 


Page. 
Measurement  of  environmental  conditions  af- 
fecting vegetation— Continued. 

Special    observations    on    climate    and    soil  of 
locality — Continued . 

Atmospheric  humidit y 

Instruments 

Wind  movement - 

Instruments 

Evaporation 151 

Objects  and  nature  of  evaporation  meas- 
urements   151 

Instrumental  methods 154 

Free-water  surface. 154 

Measurement - 154 

Nonfree-water  surface 155 

Piche  evaporimeter 155 

Porous  cup  atmometer 156 

shive's    nonabsorbent    porous-cup 

atmometer 157 

st  andardization 159 

Computation  of  field  results 159 

Exposure 160 

Forest  Service  evapori meters 161 

Observations 162 

Tabulation 163 

Direct  transpiration  method 164 

Cobalt-chloride  method 164 

Method  of  excised  twigs 165 

Method  of  potted  plants 166 

Instruments ■  168 

Phenology 168 

External  field  observations 170 

Internal  or  physiological  observations 171 

Field  observations,  photographs,  and  maps. .  172 

Appendices 175 

A.  Vapor  pressure    tables    for  barometric 

pressure  21.42  inches • 175 

B.  Osmotic   pressures   and    freezing-point 

depressions 198 

C.  Titration   methods   for   alkalinity   and 

acidity 199 

Alkalinity  test 199 

Acidity  test 200 

List  of  references 201 


INTRODUCTION. 


OBJECT. 

Forestry,  like  engineering  or  medicine,  is  largely  an  applied  science. 
Its  development  is  based  on  fundamental  knowledge  of  the  natural 
sciences.  Knowledge  of  the  tree  itself,  is  purely  botanical  and 
physiological  science.  The  first  contact  with  its  enemies  and  biotic 
aids  leads  into  mycology  and  zoology.  Investigation  of  the  effect 
of  environment  upon  the  tree  necessarily  involves  consideration  of 
geology  and  soils,  physics  and  chemistry,  climatology  and  sola 
radiation,  as  well  as  the  biology  of  the  tree's  living  companions. 
In  measuring  the  volume  and  growth  of  tree  and  stands,  as  well 


RESEARCH  METHODS  IN  STUDY  OF  FOREST  ENVIRONMENT.  3 

many  of  the  conditions  within  and  without  the  tree,  there  is  need 
for  mathematics  somewhat  beyond  the  elemental.  And  so  on  ad 
infinitum.  The  present-day  forester  is  keenly  alive  to  the  need  for 
help  from  every  possible  source  of  scientific  information. 

Unfortunately,  the  investigations  undertaken  by  those  trained  in 
forestry  must  cover  so  wide  a  field,  and  are  so  often  governed  by 
some  practical,  economic,  and  immediate  necessity,  that  there  is  no 
time  or  opportunity,  and  often  a  lack  of  the  necessary  training,  for 
delving  into  the  fundamental  problems  of  the  underlying  sciences. 
It  is  therefore  in  keeping  with  the  needs  of  forestry  and  the  spirit 
of  the  times  to  call  the  attention  of  scientists  in  every  line  to  the 
problems  that  confront  foresters  and  to  seek  the  cooperation  of  such 
scientists  in  solving  them. 

AYhile  the  present  bulletin  is  designed  primarily  for  the  aid  of 
forest  investigators — those  who  are  giving  all  of  their  time  to  for- 
estry— it  is  hoped  that  it  will  be  suggestive  to  a  great  many  others 
of  problems  well  worthy  of  their  serious  study.  An  effort  must  be 
made  to  show  to  such  workers  the  ways  in  which  forestry  is  weak 
and  as  exactly  as  possible  the  nature  of  the  problems  with  which 
foresters  are  confronted.  To  trained  scientific  workers  the  discus- 
sion of  methods  with  which  they  are  already  more  than  familiar 
will  seem  unnecessary.  To  others  familiar  with  the  problems  of  for- 
estry and  perhaps  almost  overwhelmed  by  their  magnitude  it  is  hoped 
the  same  discussions  may  bring  needed  suggestions  of  a  technical 
nature. 

A  method  of  investigation  is  to  the  scientist  what  a  tool  is  to  a 
mechanic.  The  point  of  view  of  the  investigator,  determined  by  his 
past  experience,  knowledge  of  facts,  and  philosophy,  is  to  him  what 
manual  skill  is  to  the  mechanic.  The  investigator,  like  the  mechanic, 
to  be  thoroughly  effective,  must  be  able  on  occasion  to  make  new 
tools  for  new  and  special  purposes. 

Any  suggestion  of  a  handbook,  presenting  cut-and-dried  methods 
by  which  research  is  to  be  conducted,  would  be  repugnant  to  the  true 
investigator.  The  aim  of  this  bulletin  must  be  to  clarify  the  prob- 
lems so  that  the  investigator  may  readily  choose  for  himself  the 
method  of  approach,  and  not  so  much  to  recommend  as  to  enumerate 
methods  and  equipment,  describing  their  past  accomplishment-.  If 
the  following  discussions  do  not  hold  strictly  to  this  point,  it  should  be 
understood  that  it  is  the  purpose  of  this  bulletin  to  build  a  founda- 
tion for  the  future  on  the  experience  of  the  past,  and  to  suggest  the 
form  of  the  superstructure  rather  than  its  architectural  design;.  In 
this  way  it  is  hoped  to  save  the  actual  designers  much  needless  and 
fruitless  effort. 


4  BULLETIN   105!>,   U.    S.    DEPARTMENT   OF   AGRICULTURE. 

SCOPE. 

In  surveying  the  present  field  of  forest  investigations  and  analyzing 
the  factors  which  enter  into  the  problems  and  the  methods  available 
for  their  solution,  it  appears  that,  although  the  number  of  problems 
is  great  and  they  may  vary  in  character  from  region  to  region  and 
from  period  to  period,  theoretically  they  may  be  conceived  as  falling 
into  two  essential  groups.  These  two  groups  are  (1)  ecological  and 
(2)  statistical.  In  solving  the  ecological  problems  the  aim  is  to 
express  relations;  in  solving  statistical  problems  the  aim  is  to  express 
the  bare  facts  of  forest  growth. 

This  bulletin  will  be  concerned  wholly  with  ecological  forest 
studies.  1  To  some  it  may  seem  strange  that  the  word  "ecological'' 
should  be  used  rather  than  the  more  inclusive  " biological."  The 
choice  is  a  question  of  aims  and  objectives.  " Ecological"  better 
expresses  the  objects  of  the  knowledge  foresters  seek  to  gain.  The 
practice  of  forestry  is  in  a  very  large  degree  the  application  of  ecology. 
As  an  example,  a  forester  may  be  only  slightly  interested  in  the 
abstract  physiological  fact  that  trees  require  sunlight  for  their 
development.  This  fact  is  taken  as  a  matter  of  course  and  allowed 
for.  "When,  however,  he  finds  that  one  of  two  species  with  which  he  is 
dealing  requires  much  more  sunlight  than  the  other,  or,  in  other 
words,  does  not  react  so  readily  to  the  stimulus  of  sunlight,  the 
forester  then  finds  a  keen  interest,  because  it  is  a  practical  interest, 
in  this  ecological  factor  and  its  relations. 

Or,  again,  the  matter  may  be  expressed  in  this  way:  The  forester, 
in  dealing  with  a  given  species,  feels  that  he  is  dealing  with  a  bio- 
logical entity  whose  characters  he  may  know  minutely  or  generally 
but  which  he  can  not  change,  except  possibly  through  long-term 
breeding.  On  the  other  hand,  the  environment  of  this  entity  can 
to  a  considerable  degree  be  controlled,  and  its  reactions  to  changes 
in  environment  can  be  observed.  His  concern  is  therefore  not  with 
the  physiological  functioning  in  relation  to  a  given  environment. 

Control  of  environment  is  the  cornerstone  of  the  practice  of  for- 
estry.    The  art  of  the  forester  is  primarily  the  art  of  utilizing  to  best 

1  The  statistical  group  of  problems,  in  distinctions  from  the  ecological,  includes  chiefly  those  which  deal 
with  the  determination  of  the  amount  standing  timber,  its  increment,  and  other  quantitative  changes  in 
the  stand,  with  only  general  reference  to  the  conditions  governing,  such  as  might  be  met  in  the  use  of  arbi- 
trary site  quality  classes.  As  a  matter  of  fact,  there  can  be  no  sharp  line  between  ecological  and  statistical 
foiest  studies,  and  as  forestry  advances  there  will  be  a  tendency  to  consider  all  growth  in  its  ecological  rela- 
tion is,  however,  at  the  outset  necessary  to  recognize  certain  standard  methods  for  the  measurement 
of  growth,  whatever  their  purpose  or  use.  These  methods  are  distinct  from  the  processes  which  are  ordi  - 
narily  considered  as  essential  to  progress  in  ecology,  and  it  is  for  this  reason  that '  'measurements,' '  or  statis- 
tical studies,  are  not  included  in  the  present  discussion.  The  method  of  determining  the  growth,  volume, 
and  yield  of  forest  stands  is  largely  mechanical,  though  for  sound  progress  it  should,  of  course,  involve  knowl- 
edge of  biology  as  well  as  mathematics.  The  caliper,  hypsometer,  scaling  stick,  log  scale,  increment  borer, 
and  tape  are  practically  all  the  instruments  that  are  required. 


RESEARCH  METHODS  IN  STUDY  OF  FOREST  ENVIRONMENT.  D 

advantage  the  biological  forces  active  in  forest  growth,  through  his 
ability  to  modify  the  environment.  Any  considerable  use  of  forests 
means  interference  with  the  natural  conditions  and  modification  of 
some  of  the  environmental  factors,  the  sum  total  of  which  determined 
the  character  of  the  present  forest.  Forestry  adapts  this  interference 
to  produce  the  best  results,  from  the  standpoint  of  human  needs. 
Therefore  it  has  been  thought  best  in  this  bulletin  to  take  up  each 
of  the  environmental  factors  separately,  and  to  introduce  only  such 
a  discussion  of  physiological  facts  as  seems  necessary  to  a  proper 
conception  of  the  methods  of  study  of  the  environment. 

Ecological  forest  studies  deal  with  all  problems  which  involve  the 
determination  of  the  effect  of  environmental  conditions  on  repro- 
duction, initiation,  growth,  and  physiological  functions.  To  this 
group  belong  such  studies  as  the  seed  production  of  different  species 
in  different  seasons  and  conditions;  the  characteristics  of  seeds  as 
related  to  their  origin;  the  correlation  between  the  composition,  suc- 
cession, and  growth  of  forest  vegetation  on  the  one  hand,  and  the 
conditions  of  the  environment  on  the  other;  the  vast  field  of  prob- 
lems in  natural  reproduction  and  methods  of  cutting  for  definite 
silvicultural  purposes;  the  various  phases  of  forestation,  including 
the  germination  of  seed,  requirements  for  shade  and  water  of  the 
different  species,  the  planting  of  forest  trees,  and  their  competition 
for  moisture  and  light  with  herbaceous  and  shrubby  vegetation;  and 
many  similar  problems.  The  methods  and  instruments  available  for 
the  study  of  the  ecological  forest  problems  are  essentially  the  same 
as  those  which  are  used  in  the  study  of  the  physiology  and  ecology 
of  plants  in  general.  They  involve  the  measurements  of  such  aerial 
conditions  as  precipitation,  air  temperature,  the  evaporating  power 
of  the  air,  wind  velocity  and  wind  direction,  and  sunshine  intensity; 
and  such  subterranean  conditions  as  temperature  and  moisture  of 
the  soil,  its  depth,  structure,  and  chemical  composition.  The  func- 
tioning of  the  trees  in  response  to  these  conditions  must  also  be  meas- 
ured by  the  means  recognized  and  used  by  plant  physiologists.  The 
methods  and  instruments  used  in  physiology,  meteorology,  and  soil 
physics,  therefore,  are  applicable  in  a  large  measure  to  the  study  of 
ecological  forest  problems,  though  often  with  modifications  necessi- 
tated by  the  character  of  the  plant  and  of  its  environment, 

While  it  is  true  that  in  studying  the  present  composition  of  a 
forest  stand  it  is  necessary,  to  a  certain  extent,  to  have  the  historical 
viewpoint  in  order  to  determine  clearly  how  this  stand  was  initiated 
and  why  it  now  supports  one  dominant  species  rather  than  another, 
still  it  must  be  recognized  that  historical  studies  and  conjectures  are 
outside  the  main  domain  of  ecology.  The  purpose  of  ecology  as  an 
exact  science  must  always  be  to  measure   present  conditions  and 


6  BULLETIN    10 "fl,    IT.    S.    DEPARTMENT   OF   AGRICULTURE. 

their  reactions  on  the  organism,  reducing  to  precise  terms  relations 
between  environment  and  life  which  may  be  already  understood  in 
general  terms.  In  such  processes  no  distinction  will  be  made  between 
a  condition  which  is  a  direct  result  of  the  climate  or  site,  one  which 
is  the  result  of  cumulative  effects  of  the  presence  of  the  plant  forma- 
tion, and  one  which  may  represent  the  current  influence  of  the  pres- 
ent plant  formation.  Thus,  while  recognizing  in  principle  and  in 
the  application  of  results  historical  conditions  and  the  so-called 
social  relations  which  are  particularly  important  in  forest  aggrega- 
tions, it  must  be  clearly  understood  that,  in  the  current  measure- 
ments with  which  this  bulletin  has  to  deal,  the  source  of  a  given  con- 
dition has  no  bearing  on  the  method  of  its  determination. 

SAMPLE  PLOT  METHOD  COMMON  TO  BOTH  ECOLOGICAL  AND  STATISTICAL  STUDIES. 

A  method  common  to  both  ecological  and  statistical  problems  is 
the  method  of  sample  plots.  The  details  of  the  sample  plot  method 
vary  with  the  purpose  of  the  problem  which  is  being  investigated. 
The  plot  may  vary  in  size  from  a  square  foot  to  an  entire  section. 
It  may  have  all  possible  geometrical  forms — circle,  square,  quad- 
rangle, strip,  or  triangle.  It  may  be  used  in  the  study  of  herbaceous 
vegetation,  of  seedlings  in  a  nursery,  or  of  virgin  forests;  for  the 
purpose  of  studying  the  evolution  of  the  vegetation,  for  bringing  out 
the  effect  of  a  definite  condition,  for  determining  the  growth  of  the 
vegetation,  or  for  observing  any  other  change  that  takes  place  in 
the  plant  association,  whether  it  be  grass,  brush,  or  forest.  The  prin- 
ciple, however,  remains  everywhere  the  same;  namely,  the  use  of 
areas  representative  of  a  given  type  of  vegetation  for  intensive  obser- 
vation over  a  long  period  of  time. 

NEED   FOR   A   PERMANENT   ORGANIZATION   IN    FOREST   INVESTIGATIONS. 

The  great  variety  of  forest  stands,  the  difference  between  stands  in 
different  regions,  and  the  longevity  of  trees  make  it  difficult  for  an 
individual  to  complete  any  investigation  on  the  life  of  the  forest. 
This  difficulty  is  now  universally  recognized.  A  permanent  organi- 
zation charged  with  such  investigations  has  been  formed  in  practi- 
cally every  country  in  which  the  care  of  the  forests  is  a  matter  of 
national  concern.  This  permanent  organization  consists  of  investi- 
gators assigned  to  forest  experiment  stations. 

FOREST   EXPERIMENT   STATIONS. 

Since  it  is  practically  impossible  to  follow  all  the  changes  which 

take  place  in  a  stand  during  its  entire  life  of  100  years  or  more,  the 

usual  procedure  is  to  carry  on  a  number  of  observations  simultane- 

By  distributing  the  observations  over  stands  of  the  same 


RESEARCH  METHODS  IX  STUDY  OF  FOREST  ENVIRONMENT.  7 

character,  representing  a  large  number  of  age  gradations,  the  entire 
100-year  cycle  of  development  of  the  stand  may  be  encompassed  in 
20  years.  Even  then  it  often  happens  that  a  forest  stand,  because 
of  an  accident,  such  as  fire  or  insect  infestation,  may  become  un- 
suitable for  further  observations.  It  is  evident,  therefore,  that  for 
reliable  silvicultural  conclusions  it  is  necessary  to  have  under  obser- 
vation a  large  number  of  forest  stands  for  long  periods  of  time,  and, 
therefore,  a  permanent  investigative  organization,  which  will  insure 
the  completion  of  long-term  experiments  and  correlate  in  a  sys- 
tematic and  uniform  way  the  observations  conducted  by  many  in- 
vestigators throughout  the  country.  The  investigations  which  come 
as  a  general  rule  distinctly  under  the  work  of  forest  experiment 
stations  are:  (1)  Forest  meteorological  observations;  (2)  distribu- 
tion of  species  and  types  in  relation  to  climate  and  soils ;  (3)  studies 
of  the  growth,  volume,  and  yield  of  forest  stands;  (4)  studies  of 
the  effect  of  the  source  of  seed  upon  the  resulting  forest  stand;  (5) 
experiments  with  the  introduction  of  exotic  species;  (6)  experiments 
with  different  silvicultural  methods^  of  cutting  for  the  purpose  of 
securing  natural  reproduction;  j^)"^ethods  of  artificial  reproduc- 
tion; (8)  the  study  of  the  ei^:\^different  methods  of  thinning 
upon  the  growth  of  the  mai^stajaa;  and  (9)  studies  of  the  effect  of 
site  upon  the  technical  properties  of  the  wood  produced.  These 
investigations  are  beyond  the  ability  of  an  individual  investigator 
to  handle  because  their  solution  requires  either  a  very  long  period 
of  years,  often  exceeding  the  life  of  a  single  man,  or  the  simultane- 
ous establishment  of  many  experiments  in  different  places — a  whole- 
sale method  of  observations — or  expensive  apparatus.  It  is  true 
that  some  of  the  problems  involved  have  been  studied  by  individual 
investigators  with  very  suggestive  results,  but  there  is  no  doubt  that 
forest  experiment  stations,  being  less  subject  to  the  uncertainties  of 
individual  effort,  can  conduct  such  studies  with  greater  uniformity 
and  assurance  of  success. 

SHORT-TERM    STUDIES. 

Although  in  the  study  of  forest  stands  the  most  reliable  results  will 
be  secured  only  by  permanent,  well-equipped  experiment  stations 
organized  and  maintained  by  the  Federal  Government,  States,  or 
institutions,  much  can  be  accomplished  also  by  comparatively  short 
studies  of  individual  investigators. 

Studies  which  do  not  involve  continuous  observations  for  a  long 
period  of  years  or  expensive  stationary  instruments  and  equipmentr- 
for  example,  microscopic  and  chemical  studies  of  woods  or  studies  of 
natural  reproduction,  distribution,  and  growth— may  be  conducted 
without  permanent  forest  experiment  stations;  and  even  observations 


8  BULLETIN   1059,   U.    S.    DEPARTMENT   OF   AGRICULTURE. 

on  climate  in  its  relation  to  forest  vegetation  may  sometimes  be  made 
on  short  field  trips.  Very  often  the  painstaking  observer,  without 
extensive  apparatus,  will  discover  some  fundamental  facts  which  alter 
the  conception  of  a  given  problem,  and  which  therefore  lead  to  far 
more  productive  efforts  by  the  permanent  organizations  whicn  can 
study  the  problem  for  longer  periods.  It  is  only  by  recognizing  this 
principle  of  supplemental  effort  that  substantial  progress  in  forest 
investigations  can  be  made.  There  should  be  no  attempts  to  delimit 
the  work  of  any  organization  or  individual. 

THE   SIMPLE   PHYSICO-PHYSIOLOGICAL   CONCEPT. 

Many  ecological  problems  are  less  confusing  to  the  beginner  and 
are  more  likely  to  be  approached  by  sound  methods  if,  at  the  outset,  a 
rather  definite  physical  interpretation  of  life  is  accepted,  for  through 
such  a  concept  is  gained  an  idea  as  to  the  probable  physical  reaction 
to  the  environment  and  the  method  of  measuring  the  physical  con- 
ditions. 

Thus,  to  begin  with,  the  living  mass  of  plants  (the  protoplasmic 
mass,  primarily)  may  be  conceived  to  be  simply  a  colloidal  mass  of 
organic  compounds  with  a  peculiar  affinity  for  water.  Water  is  of 
fundamental  importance  to  its  life  qualities.  To  supply  the  demand 
for  water,  the  protoplasmic  mass  must  posses  a  greater  affinity  for  it 
than  the  soil  or  solution  from  which  the  water  is  to  be  obtained.  The 
struggle  for  water  is,  primarily,  a  contest  between  the  colloids  of  the 
plant  and  the  organic  and  inorganic  (clay)  colloids  of  the  soil. 

Secondly,  it  is  inevitable  that  any  object  possessing  water  should 
lose  the  same  by  evaporation  to  the  atmosphere  until  a  balance  is 
reached  between  the  vapor  pressure  of  the  water-holding  mass  and 
that  of  the  atmosphere.  Such  an  equilibrium  does  not,  necessarily, 
mean  death,  at  least  for  certain  kinds  of  tissues,  but  the  small  supply 
of  water  represented  by  equilibrium  with  ordinary  atmospheric  vapor 
is  insufficient  to  permit  photo-synthesis,  metabolism,  and  transport 
within  the  plant.  For  continued  functioning,  the  plant  must  be  able 
to  maintain  its  water  supply  above  this  level. 

The  objective  of  physiological  functioning  is  reproduction,  to  which 
growth  is  only  incidental.  The  object  in  the  existence  of  any  indi- 
vidual plant  is  to  extract  enough  phosphorus1  from  the  soil  so  that  a 
peculiar  accumulation  of  matter  called  a  seed  may  be  formed,  witli 
a  sufficient  affinity  for  water  and  a  sufficiently  close  chemical  combi- 
nation to  enable  this  embryonic  plant  to  resist  all  of  the  forces  of 
disintegration  during  a  period  of  dormancy. 

»  Phosphorus  is  mentioned  only  as  an  example  of  the  vitally  necessary  elements  obtained  from  the  soil. 


RESEARCH  METHODS  IN  STUDY  OF  FOREST  ENVIRONMENT.  9 

The  first  requirement,  then,  is  that  the  present  plant  should  live 
long  enough  to  accumulate  by  absorption  from  the  soil  a  quantity  of 
phosphorus  which  may  be  concentrated  in  this  one  seed,  or  ten 
thousand  seeds,  as  the  case  may  be. 

To  accomplish  this  object,  it  is  rather  evident  that  a  large  amount 
of  water  must  be  absorbed  and  disposed  of,  with  a  resulting  deposit 
of  phosphorus  and  other  solids  as  the  water  is  evaporated.  Even 
then  there  must  be  a  strong  tendency  for  such  solids,  if  retained  in 
solution,  to  diffuse  back  to  the  roots  and  into  the  soil.  Not  denying 
the  possible  ability  of  the  plant  to  trap  and  hold  phosphorus,  or  any 
other  needed  substance,  at  the  point  where  needed,  it  seems  necessary 
to  call  into  play  some  other  physical  force  to  effect  this  concentration. 
The  only  other  possible  force  is  the  electromagnetic  affinity  of  energy 
for  matter  and  of  matter  for  energy.  The  ability  of  the  plant  to 
concentrate  the  essential  inorganic  substances  in  the  best-lighted 
parts  of  its  structure  may  thus  be  explained. 

In  other  words,  the  requirement  of  plants  for  light  is  primarily  a 
requirement  for  a  concentration  of  essential  substances  needed  for 
reproduction.  But  light  can  only  be  obtained  where  there  is  com- 
petition through  growth.  To  insure  the  necessary  amount  of  light 
the  individual  plant  is  required  to  keep  its  head  at  least  up  to  the  level 
of  the  competitors,  and  the  plant  which  becomes  dominant  is  most 
certain  to  reproduce.  Possible  differences  between  plants,  in  their 
ability  to  make  use  of  different  kinds  of  light,  need  not  be  discussed 

here. 

So,  then,  reproduction  requires  light,  the  need  for  light  calls  for 
growth,  and  growth  in  turn  is  possible  only  through  the  action  of 
light  in  photo-synthesis,  or  the  creation  of  new  organic  matter  by  the 
combination  of  water  and  carbon  dioxide. 

This  necessary  combination  of  water  from  the  soil  and  carbon 
dioxide  from  the  air  can  be  effected  only  by  exposing  the  cells  con- 
taining water  to  the  air,  so  that  the  carbon  dioxide  may  be  absorbed 
by  these  cells.  The  important  feature  ecologically  is  that  such  ex- 
posure inevitably  results  in  considerable  losses  of  water;  and  even 
though  the  cells  so  exposed  may  be  somewhat  protected,  it  is  evi- 
dent that  carbon-dioxide  absorption  and  water  loss  must,  in  a  given 
plant,  run  about  parallel,  both  being  controlled  by  the  size  of  the 
stomatal  openings.  The  actual  water  loss,  of  course,  will  vary  ac- 
cording to  the  dryness  of  the  air,  the  concentration  and  vapor 
pressure  of  the  contents  of  the  exposed  cell,  and  the  intensity  of  the 
light  in  which  the  operation  is  performed.  Thus  a  plant  capable  of 
making  use  of  diffused  light,  or  largely  of  the  so-called  actinic  rays, 
may  function  with  less  water  loss  than  one  exposed  to  the  full  heat- 
ing effect  of  sunlight. 


10  BULLETIN   1059,   U.    S.   DEPARTMENT   OF   AGBICULTUBE. 

Until  the  distribution  of  essential  substances,  such  as  phosphorus, 
in  the  plant  has  been  more  fully  and  carefully  studied  in  relation 
to  lio-ht  and  the  volume  of  the  water  stream,  it  is  impossible  to 
form  a  fair  opinion  as  to  whether  the  latter,  and  the  transpiration 
of  a  large  volume  of  water,  are  really  essential  to  the  end  for  which 
the  plant  exists.  On  this  point  botanists  have  ever  been  at  variance. 
For  the  present,  however,  transpiration  is  believed  to  be  merely 
an  unavoidable  concomitant  of  carbon-dioxide  absorption,  serving 
no  useful  purpose  when  carried  to  extremes,  while  always  menacing 
the  existence  of  the  plant. 

There  is  now  only  one  more  very  essential  point  to  be  touched 
upon — a  point  which  is  of  especial  interest  in  connection  with  the 
study  of  trees  because  of  their  perennial  character.  The  continu- 
ous absorption  of  water  at  the  roots  and  its  loss  at  the  leaves  of 
plants  is  necessarily  accompanied  by  the  absorption  of  all  salts 
which  are  contained  in  the  soil  solution.  There  is  undoubtedly  some 
so-called  selective  absorption  in  the  sense  that  any  semipermeable 
membrane  admits  the  complex  molecules  less  readily  than  the  simple 
ones,  but  the  ability  of  the  plant  to  differentiate  between  useful  and 
unnecessary  salts  is  not  admitted.  It  is  therefore  inevitable  that 
the  leaves  should  accumulate  quantities  of  material  which  can  not 
be  used;  that  there  should  be  a  tendency  for  such  materials  to  dif- 
fuse  back  toward  the  roots;  that  when  such  mat  dial  is  present  in 
sufficient  quantities  it  should  be  precipitated  or  crystallized,  and  in 
such  form  should  tend  to  obstruct  the  flow  of  water  in  the  channels 
where  it  exists.  It  is  conceivable,  then,  that  all  tissues  which  are 
actively  engaged  in  the  transport  of  water  must  eventually  heroine 
"silted  up'  with  this  useless  material  and  that  this  i>  the  cause  of 
sensecence.  Its  best  illustration  is,  perhaps,  in  the  petiole  of  the 
broad  leaf,  through  which  narrow  passage  a  large  evaporating  sur- 
face must  be  supplied.  This  conception  explains  quite  well  the 
eventual  failure  of  leaves  to  function  and  their  gradual  drying  and 
falling,  even  in  those  forms  in  which  the  leaves  are  not  in  th 
sensitive  to  seasonal  changes.  It  also,  perhaps,  explains  the  forma- 
tion of  heartwood  in  trees.  The  more  important  idea,  however,  i> 
that  it  points  to  the  necessity  for  growth  to  maintain  existence.  It 
is  not  sufficient  that  the  "suppressed'  tree  [as  every  forester  calls 
the  tree  growing  with  insufficient  light)  should  obtain  enough  wat<  r 
to  prevent  the  desiccation  of  the  foliage.  The  plant  musl  he  peri- 
odically enabled  to  produce  some  new  growth  or  it    succumbs   t<» 

fiility,  regardless  of  age.     Apparently  the  maturity  o\   a   normal 
"i-  even  a  dominant  tree  is  attained  soon  after  its  limit  of  height   is 
reached,  as  it  is  then  limited  in  its  extensions  for  light  and  soon  can 
make  the  needed  annual  additions  to  its  transporting  system. 


RESEARCH  METHODS  IX  STUDY  OF  FOREST  ENVIRON  M  E XT.         11 

It  is  hoped  that  this  discussion  will  clarify  the  point  of  view  which 
prevails  in  the  disccussion  of  the  individual  environmental  conditions. 

MEASUREMENTS  OF  ENVIRONMENTAL  CONDITIONS  AFFECT- 
ING FOREST  VEGETATION. 

The  character  of  the  forest  and  its  very  existence  are  determined 
by  the  climate,  soil,  and  subsoil  of  the  locality.  The  general  charac- 
ter of  the  region,  including  the  character  of  the  vegetation  and  of 
the  soil,  is  determined  in  the  highest  degree  by  the  climate.  The 
climate  affects  the  region  and  vegetation  in  two  ways:  (1)  It  is  at 
present  the  most  important  factor  in  the  environment  of  the  vegeta- 
tion; (2)  it  has  affected  the  present  environment  in  its  historical  de- 
velopment; for  instance,  in  the  formation  of  the  soils,  their  present 
physical  and  chemical  composition  being  largely  the  result  of  the 
past  climate  in  combination  with  other  natural  factors.  The  deter- 
mination of  the  important  features  of  a  climate  is  not  a  simple  mat- 
ter. It  must  rest  upon  a  sufficiently  long  series  of  observations  at 
well-equipped  meteorological  stations. 

CLIMATIC   CHARACTERISTICS    OF   LOCALITY. 

NATURAL     CLIMATIC     REGIONS. 

The  characteristics  of  a  climate  must  be  studied  first  of  all  by 
natural  regions  and  the  study  based  on  the  observations  of  several 
stations  located  in  different  parts  of  the  same  region.  The  climate  of 
individual  localities  may  best  be  analyzed  by  comparison  with  the 
climate  of  the  entire  natural  region  in  which  the  locality  is  found  or 
of  a  control  station  centrally  located. 

DATA    OBTAINED   BY    WEATHER   BUREAU. 

For  general  climatic  studies  of  the  forest  regions,  and  to  some 
extent  in  studying  the  conditions  for  growth  in  established  stand-. 
the  data  collected  by  the  United  States  Weather  Bureau  at  its  numer- 
ous regular  stations  may  be  used  to  good  advantage.  At  the  greater 
number  of  these  stations  only  data  on  air  temperatures  and  precipita- 
tion are  obtained.  At  the  larger  stations  data  on  humidity,  sun- 
shine, barometric  pressure,  etc.,  are  obtained,  but  because  of  the  al- 
most universal  location  of  such  stations  in  towns  and  cities  the 
applicability  of  the  data  to  forest  conditions  is  often  very  question- 
able. It  appears,  therefore,  that  the  regular  observations  ^  the 
Weather  Bureau  will  furnish  us  principally  with  precipitation  and 
temperature  data  by  which  the  broader  forest  regions  may  he 
defined.  The  use  of  these  same  data  in  strictly  local  studies  will 
depend  entirely  on  the  minute  examination  of  the  conditions  sur- 
rounding the  station. 


12  BULLETIN   1059,   U.    S.   DEPARTMENT  OF   AGRICULTURE. 

KNOWLEDGE    OF   EXISTING   WEATHER   BUREAU   STATIONS    NECESSARY. 

Before  attempting  any  meteorological  observations  the  investi- 
gator should  visit  the  nearest  permanent  meteorological  stations 
and  obtain  a  clear  understanding  of  the  manner  in  which  the  obser- 
vations are  made,  compare  his  own  instruments  with  those  of  the 
station,  and  ascertain  the  natural  conditions  in  which  the  permanent 
station  is  located  and  the  extent  to  which  they  are  typical  of  the  re- 
gion. This  is  essential  to  enable  the  investigator  to  decide  whether 
and  to  what  extent  he  would  be  justified  in  connecting  his  special 
meteorological  observations  with  those  of  the  permanent  station. 
Observations  at  permanent,  well-equipped  Weather  Bureau  stations 
are  not  always  conducted  in  the  way  that  meets  the  special  needs  of 
the  investigator.  There  may  be  observations  essential  to  the  forester 
which  are  not  being  made  at  all.  Furthermore,  the  data  of  their 
permanent  station  will  not  always  enable  one  to  judge  of  the  effect 
of  the  climatic  conditions  upon  forest  vegetation.  For  instance,  the 
measurements  of  the  temperature  of  the  air  are  always  made  at  a 
regular  Weather  Bureau  station  at  some  height  above  the  ground 
and  in  a  more  or  less  open  place  outside  of  the  forest;  while  to  the 
forester,  the  temperature  of  that  layer  of  the  air  in  which  most  of 
the  forest  vegetation  is  found  has  the  greatest  significance.  Again, 
while  a  very  precise  measure  of  precipitation  may  be  of  no  use  to 
the  investigator,  the  amount  falling  in  single  storms  may  vary  so 
greatly  in  short  distances  that  a  record  obtained  a  few  miles  away 
will  be  very  misleading.  It  is  thus  evident  that  forest  research  has 
special  meteorological  problems,  and  that  usually  the  long-estab- 
lished weatherstation  may  serve  better  as  a  control  than  as  a  definite 
point  for  obtaining  information  about  forest  conditions. 

COMPUTATION  OF  ALL  WEATHER  DATA  BY  PERIODS  OF  GROWTH  AND 

REST. 

One  essential  thing  to  be  kept  in  mind  is  that  plants  may  react 
to  the  climatic  conditions  in  altogether  different  ways  during  periods 
of  growth  and  rest.  To  analyze  the  reactions  of  plant  life  it  is 
usually  desirable,  therefore,  to  compute  climatic  data  by  such  periods. 
They  may  be  based  either  on  a  knowledge  of  the  particular  plant 
formation  wdiich  each  observation  point  represents,  or  on  the  average 
period  of  the  native  vegetation  of  the  locality.  Usually  it  will  be 
preferable  to  adopt  first  a  "growing  season"  for  the  wdiole  region 
under  study.  Later,  for  more  exact  comparison  of  the  component 
formations  and  after  careful  determination,  the  specific  period^  of 
plant  activity  may  be  employed  in  summarizing  temperatures,  etc. 


RESEARCH  METHODS  IN  STUDY  OF  FOREST  ENVIRONMENT.         13 
SPECIAL  OBSERVATIONS  ON  CLIMATE  AND  SOIL  OF  LOCALITY. 

To  obtain  concrete  information  on  restricted  localities  and  specific 
forest  types  it  will  be  necessary  in  most  instances  for  forest  investi- 
gators to  establish  apparatus  and  make  observations  independently. 
In  the  more  important  respects  the  accepted  procedure  of  meteor- 
ologists and  the  standard  instruments  may  be  used  by  the  foresl 
investigator,  but  the  latter  will  also  require  many  data  not  obtained 
in  routine  meteorological  work,  and,  especially  in  the  location  of 
instruments,  will  be  compelled  to  vary  procedure  according  to  local 
needs. 

LOCATION  OF  INSTRUMENTS  FOR  THE  STUDY  OF  THE  GROWTH  OF  FOREST  STANDS. 

Atmospheric  conditions  affecting  the  growth  of  forest  stands  a- 
a  whole  should  naturally  be  measured  at  a  distance  from  the  ground 
which  will  represent  the  mean  height  of  the  sensitive  portion  of  tl it- 
tree;  that  is,  the  mean  elevation  of  the  crown.  Thus,  if  a  stand 
were  generally  devoid  of  green  limbs  for  the  first  10  feet  of  the  stem- 
and  had  an  average  total  height  of  70  feet,  the  observations  should 

be  at  10  4 ^—  >    or  40  feet  from  the  ground.     Measurements  of  the 

li^ht  received  by  the  stand  should  obviously  be  made  at  an  elevation 
where  none  of  the  light  is  intercepted.  The  same  result  may  some- 
times be  obtained  by  measurements  near  the  ground  in  a  large  open- 
ing on  the  same  site.  Soil  conditions  should  be  measured  at  all 
depths  which  the  roots  of  the  trees  may  be  reasonably  expected  to 
reach.  The  depth  will  be  less  in  heavy  than  in  lighl  <oils.  In  gen- 
eral, however,  it  is  believed  that  an  extreme  depth  of  4  feet  is  suffi- 
cient, though  any  evidence  to  the  contrary  should  change  the  pro- 
cedure. The  rule  of  measuring  soil  temperatures  at  the  surface  and 
1  and  4  feet  may  be  followed.  If  it  should  appear  necessary  in  using 
the  data,  the  temperatures  at  other  depths  may  be  obtained  by  plot- 
ting the  known  values  and  by  interpolating  on  the  curve  which  may 
be  drawn  for  any  given  period,  assuming  the  temperature  at  20  or  30 
feet  to  be  always  equal  to  the  local  mean  annual  temperature.  Sim- 
ilarly, soil  moisture  may  be  determined  at  the  surface  and  at  1.  2,  :>, 
and  possibly  4  feet,  and,  by  projecting  the  curve  formed  by  plotting 
the  moisture  of  these  points  the  moisture  at  greater  or  intermediate 
depths  may  be  approximated. 

LOCATION    OF    INSTRUMENTS    FOR    THE    STUDY    OF    CONDITIONS    AFFECTING 

REPRODUCTION. 

It  is  only  logical  to  assume  that,  before  a  definite  plant  formation 
or  forest  type  can  be  developed,  there  must  exist  conditions  favorable 
to '  germination  and  development  of  the  small  and  very   sensitive 


14  BULLETIN   1059,   U.    S.   DEPARTMENT  OF   AGRICULTUBE. 

seedlings.     The  forester  is  often  concerned  only  with  the  problem 
of  " securing  reproduction,"  realizing  that,  once  the  seedlings  of  a 
given   species    are   established,    the   future    of    the    stand    is    quite 
definitely  assured  and  practically  beyond  his  ability  to  influent 
In  forestry  particularly,  because  perennial  plants  are  the  subjects  of 

My.  the  seedling  stage  presents  the  most  acute  practical  problems 
and  those  most  deserving  of  scientific  study.  What  bearing  this 
has  on  the  methods  to  be  followed  in  ecological  investigations  may  be 
readily  illustrated.  If,  for  example,  it  should  be  noted  that  seedlings 
of  a  given  species  die  in  great  numbers  during  their  first  or  second 
winter  and  it  is  desired  to  determine  why  such  losses  occur  and 
whether  they  are  preventable,  it  might  be  deemed  necessary  to  study 
the  rate  of  evaporation  and  the  amount  of  drying  to  which  such 
seedlings  are  subjected  during  periods  when  the  soil  is  frozen.  Obvi- 
ously, it  would  be  necessary  to  determine  this  period  precisely  and 
to  know  (1)  when  the  soil  was  frozen  throughout  the  root  zone  of 
the  seedlings,  and  (2)  when  it  was  frozen  at  the  surface  so  that 
moisture  obtained  below  might  not  reach  the  aerial  portion.  On 
the  other  hand,  the  atmospheric  stresses  and  the  tendency  toward 
evaporation  losses  generally  might  be  measured,  that  is  to  say,  for 
the  locality  and  at  a  convenient  spot;  but  it  would  be  apparent  that 
if  the  seedlings  under  observation  were  covered  by  snow  the  rate 
of  evaporation  above  that  snow  layer  would  have  no  significance 
whatever. 

The  point,  therefore,  needs  the  greatest  possible  stress  that,  m  the 
investigation  of  many  of  the  particular  problems  of  reproduction  and 
distribution  of  the  species,  the  investigator  must  be  concerned  with  tin- 
immediate  conditions  of  the  surface  soil  and  the  atmospheric  and  solar 
conditions  at  an  elevation  barely  above  the  surface  soil,  in  connection 
with  germination,  with  survival  before  the  seedling  becomes  well 
rooted,  and  with  possible  injury  through  heat  or  drought  at  the  soil's 
surface  before  the  young  stem  is  protected  by  an  effective  corticle. 
Measurements  at  depths  of  even  1  foot. in  the  soil,  or  at  elevations  of  a 
foot  above  it,  will  usually  only  be  made  to  give  general.  Comparative 
indications  of  the  conditions  which  it  is  really  neces>ary  to  under- 
stand; and  because  the  rapidly  fluctuating  conditions  of  the  soil's 
surface  are  in  many  ways  extremely  difficult  to  cope  with. 

In  considering  the  conditions  which  affect  reproduction,  an  eleva- 
tion of  G  inches  above  the  surface  may  possibly  be  accepted  as  the 
lowest  level  at  which  aerial  measurements  are  practicable,  hut  by  the 
exercise  of  ingenuity  it  should  be  possible  to  improve  on  this.  Ln  soil 
study,  greatest  attention  must  be  paid  to  the  near-surface  conditions. 
The  actual  moisture  of  the  covering  of  litter  and  humus,  as  well  as 
that  of  the  first  mineral  soil,  is  obviously  important,  but ' extremely 


RESEARCH   METHODS  IN  STUDY  OF  EOREST  ENVIRONMENT.         15 

difficult  to  measure  with  any  great  accuracy,  especially  as  the  humus 
layer  is  seldom  the  same  on  any  two  spots  which  might  be  selected, 
and  may  change  in  moisture  content  almost  as  rapidly  as  the  atmos- 
phere. It  is  almost  inevitable,  therefore,  that  actual  moisture  meas- 
urements should  be  confined  to  the  first  layer  of  mineral  soil  and  to 
greater  depths  if  desired,  and  that  .the  depth  and  character  only  of 
the  humus  should  be  noted,  using  some  predetermined  rule  for  esti- 
mating its  moisture  content  at  various  times.  For  soil  temperatures 
conditions  at  the  surface  are  doubtless  of  the  greatest  importance; 
but  here  again  the  measurement  of  the  actual  and  constantly  change 
ing  soil  conditions  present  a  practical  difficulty.  Measurements  be- 
low the  surface  may  have  considerable  comparative  value,  even 
though  they  do  not  give  the  extremes  which  may  have  the  most  direct 
bearing  on  plant  life;  and  it  is  therefore  suggested  that  a  depth  of 
1  foot  be  taken  in  all  such  studies  as  furnishing  a  kind  of  control 
for  other  observations. 

Having  considered  the  general  arrangement  of  apparatus,  the  mat- 
ter of  exact  methods  and  instruments  to  be  used  in  measuring  each 
aerial  and  soil  condition  may  now  be  taken  up. 

AIR    TEMPERATURES. 

Air  temperatures  are  more  readily  measured  than  any  other  con- 
dition because  of  the  simple  equipment  required,  and  they  will  prob- 
ably be  most  frequently  considered  at  temporary  stations.  It  is 
hardly  to  be  questioned  that  air  temperatures  affect  growth  very 
directly,  although  this  may  not  always  be  apparent  if  only  periodic 
and  annual  mean  temperatures  are  considered.  It  is  also  fairly  ap- 
parent that  the  air  temperature  which  is  adequate  for  the  growth 
of  an  individual  plant  receiving  an  abundance  of  light  may  he  quite 
inadequate  for  one  growing  in  competition  with  or  in  the  shade  of 
other  plants.  Then  there  are  the  maximum  temperatures  to  he  con- 
sidered, which  it  now  seems  may  be  more  directly  operative  in  pre- 
venting the  extension  of  plant  ranges  than  any  other  temperature 
condition.  In  this  connection,  the  temperature  of  the  soil  surface 
may  be  most  important,  but  that  of  the  air  layer  just  above  the  soil 
must  not  be  overlooked. 

The  following  problems  summarize  briefly  what  are  believed  to  be 
the  most  important  temperature  problems  in  relation  to  forestry. 

Problkm^ 

1.  Temperature  zones,  as  indicated  by  mean   monthly,  seasonal, 
and  annual  air  temperatures,  or  length  of  frpstless  season,  or  tem- 
perature sums  (hour-degrees)  above  a  fixed  minimum  (say,    to     1 
which  furnish  the  conditions  necessary  for  the  existence  oi   a  given 
species. 


16  BULLETIN   1059,   L\    S.   DEPARTMENT  OF   AGRICULTURE. 

2.  Actual  rate  of  growth  in  height,  diameter,  volume,  or  weight  of 
any  species,  within  different  temperature  limits,  should  preferably  be 
determined  under  controlled  conditions  of  moisture  supply  and  sun- 
light. 

3.  Especially  in  connection  with  the  preceding,  and  as  integrating 

air  temperature  and  sunlight  influences,  leaf  temperatures  should  be 
measured  as  a  more  direct  criterion  of  the  temperature  conditions 
regulating  food  production  and  growth.  What  is  particularly 
souo-ht,  of  course,  is  the  relation  between  leaf  temperatures,  air  tern- 
peratures,  and  sunlight,  and  whether  or  not  this  is  essentially  dif- 
ferent in  different  plants.  It  is  probably  necessary  to  determine  gen- 
eral relations  of  this  kind  and  to  base  observations  of  growth  on  long- 
term  air  temperature  records. 

4.  The  maximum  temperatures  which  may  be  tolerated  without 
highly  destructive  reactions  in  the  plant,  leading  to  fatal  results,  have 
been  investigated  very  little  and  apparently  have  received  very  little 
weight  in  considering  problems  of  distribution,  although  a  number 
of  investigators  have  shown  that  growth  rate  falls  off  rapidly  beyond 
a  certain  optimum  temperature.  The  difficulty  of  observations  on 
this  point  lies  in  the  extremely  close  connection  which  is  likely  to 
exist,  under  any  natural  conditions,  between  very  high  tcmpemnires 
and  excessive  transpiration  or  positive  drought  in  the  soil's  surface. 

In  the  general  study  of  climatic  or  temperature  zones  affecting 
plant  distribution  and  life  forms,  Merriam's  (30)2  work  has  become 
classic.  The  more  minute  determination  of  forest  zones  may  begin 
with  comparison  of  mean  temperatures  or  temperature  sums  above  a 
minimum  of  about  40°  F.,  or  similar  sums  for  the  frostless  period. 
Livingston  (25),  in  a  general  survey  of  the  temperatures  of  the 
United  States,  carried  the  matter  one  step  farther  by  rating  the  chem- 
ical efficiency  of  temperatures  above  40°  F.,  according  to  the  van't 
Hoff-Arrhenius  principle  of  doubled  activity  for  each  18°  F.  increase 
in  temperature.  These  temperature  efficiencies  were  then  summed 
for  the  growing  season,  in  place  of  the  original  temperatures.  Samp- 
son (32),  McLane  (31),  and  Lehenbauer  (24),  have  tried  various 
modifications  of  the  Merriam  idea  of  temperature  sums,  all  of  which 
should  be  looked  into.  One  will  hardly  escape  the  conviction  that  the 
consideration  of  any  temperature  term  other  than  the  mean  tempera- 
ture will  require  the  accumulation  of  hourly  temperature  records  or. 
in  other  words,  the  use  of  the  thermograph. 

In  the  more  exact  study  of  the  rate  of  growth  as  influenced  by 
temperature  a  greater  number  of  technical  problems  are  presented. 
The  temperature  coefficient  can  not  be  determined  unless  moisture  and 
sunlight  are  under  control.  The  actual  measurement g  of  growth 
rate  are  difficult,  and  necessitate  first  of  all  plants  of  uniform  aj 

*  The  figure?  in  parentheses  refer  to  the  bibliography  at  the  end  of  this  bulletin. 


RESEARCH  METHODS  IX  STUDY  OF  FOREST  ENVIRONMENT.         17 

and  size  for  the  various  comparisons.  It  seems  to  be  quite  well  estab- 
lished that  growth  of  most  plants  begins  at  about  40°  F.,  is  very  slow 
up  to  about  60°  F.,  and  reaches  a  maximum  at,  perhaps,  80°  F.  These 
points  will  be  at  least  suggestive  of  the  temperature  ranges  and  tem- 
perature groups  to  be  considered.  It  will,  however,  probably  not  be 
satisfactory  to  merely  note  that  a  given  growth  was  secured  in  A 
hours  in  temperatures  between  T  and  Tr  MacDougal  (27)  sug- 
gests the  summation  of  temperatures  from  what  may  be  called  the 
base  of  each  temperature  range  (say,  about  60°  F.,  but  not  more  than 
65°  F.),  and  as  the  simplest  means  of  obtaining  hour-degrees  in  each 
temperature  range  has  used  the  planimeter  to  measure  the  area  on 
thermograph  traces  included  between  any  two  given  lines. 

The  study  of  leaf  temperatures  is  not  a  study  of  the  environment, 
but  will  be  at  least  a  means  to  a  better  understanding  of  the  action 
of  the  environment  and  will,  perhaps,  lead  to  more  comprehensive 
and  expressive  measurements  of  the  environment.  A  good  deal  of 
rough  work  has  been  done  in  measuring  the  temperatures  of  leaves 
usually  by  wrapping  them  about  the  bulb  of  a  thermometer  or  placing 
the  latter  in  close  contact  with  them.  Such  methods,  however,  are 
wholly  inadequate  for  treating  the  needles  of  conifers,  and  are  of 
doubtful  value  elsewhere.  E.  Shreve  (36)  has  made  use  of  the  great 
sensitivity  and  possibly  small  bulk  of  a  thermocouple  to  devise  an 
apparatus  which  will  readily  reflect  the  temperature  of  any  part  of  a 
leaf  with  which  it  is  brought  into  contact.  The  whole  equipment 
seems  sufficiently  compact  and  practical  to  furnish  great  usefulness 
in  the  field  as  well  as  in  the  laboratory. 

With  this  sketchy  consideration  of  the  problems  which  should  be 

faced,  the  ordinary  means  of  accumulating  temperature  records  may 

now  be  mentioned. 

Exposure  of  Thermometers. 

Comparisons  of  air  temperatures  under  different  conditions  can, 
of  course,  be  made  only  if  the  measurements  are  made  in  such  a 
manner  as  to  eliminate  radiation  influence.  Radiation  measure- 
ments or  "sun  temperatures'1  undoubtedly  have  their  places,  but 
are  not  to  be  confused  with  the  present  subject   and   they  will    be 

discussed  later. 

To  measure  correctly  the  temperature  of  the  air.  direct  or  reflected 
sunlight  must  be  excluded  from  thermometeis  as  fully  a-  possible. 
At  the  same  time,  the  shelter  which  affords  this  protection  nxusl  not 
itself  absorb  the  radiation  sufficiently  to  become  heated  within. 
This  danger  is  largely  overcome  by  allowing  free  circulation  of  air 
through  the  shelter,  and  the  danger  is  still  further  lessened  when 
the  air  circulation  is  naturally  strong.  Such  radiation  is  particu- 
larly to  be  guarded  against  in  any  kind  of  shelter  placed  on  or  near 
10163— 22— Bull.  1059 2 


18  BULLETIN   1059,   U.   S.   DEPARTMENT  OF   AGRICULTURE. 

the  ground.  The  standard  type  of  shelter  is  double-roofed  and  has 
a  partly  open  floor,  and  walls  made  of  slats  which  overlap  to  ex- 
clude light  from  any  overhead  source  without  causing  complete 
stagnation  of  the  air  within. 

Modifications  which  would  still  more  fully  overcome  the  heating 
of  the  shelter  have  been  proposed  by  Koppen  (23),  who  would  pro- 
vide artificial  air  circulation,  but  such  provisions  will  hardly  be 
necessary  for  any  ordinary  thermometric  work.  On  the  other  hand, 
the  observer  can  not  be  too  strongly  urged  to  provide  shelters  which 
will  give  the  maximum  of  light  protection  without  preventing  the 
natural  air  currents  from  coming  in  contact  with  the  thermometers. 
To  obtain  true  nocturnal  temperatures  near  the  ground  it  may  be 
desirable  to  use  a  shelter  without  a  floor,  so  that  radiation  from 
below  the  thermometers  is  retarded  as  little  as  possible.  In  the 
work  at  the  Fremont  Experiment  Station,  Bates  has  found  that  a 
shelter  for  ground  temperatures  need  be  no  more  than  a  hood,  fully 
open  to  the  north  and  below  the  thermometers.  If  there  should  be 
considerable  reflection  from  the  north  at  midday,  it  could  be  largely 
eliminated  by  an  absorbing  screen  set  a  foot  or  two  from  the  hood. 

Standardizing  Thermometers. 

The  present  possibilities  of  correlation  between  temperatures  and 
plant  behavior  do  not  justify  the  greatest  precision  in  thermometry. 
Units  smaller  than  1°  may  be  ignored  in  field  work,  for  all  practical 
purposes,  though  personal  taste  may  dictate  that  tenths  of  degrees 
be  recorded.  The  essential  thing  is  that  only  reliable  thermometers 
be  used,  as  the  errors  in  cheap  thermometers  are  not  uncommonly  as 
great  as  the  difference  between  two  conditions  which  are  being 
studied.  Even  the  standard  types  of  maximum,  minimum,  and  mer- 
curial thermometers  may  well  be  critically  examined  and  compared 
with  a  standard  before  being  used.  The  Bureau  of  Standards  (37) 
calibrates  such  instruments  at  a  nominal  cost. 

With  recording  apparatus,  such  as  the  air  thermograph,  adjust- 
ment takes  the  place  of  standardization.  The  use  of  any  such  re- 
corder, without  thermometers  to  check  its  accuracy  at  frequent  in- 
tervals, can  not  be  recommended. 

Maximum,  Minimum,  and  Current  Temperatures. 

Where  only  maximum  and  minimum  thermometers  are  available, 
of  the  standard  Weather  Bureau  pattern,  the  maximum  and  mini- 
mum temperatures  for  the  preceding  24  hours  should  be  recorded 
once  each  day,  either  before  10  a.  m.  or  after  4p.m„  and  at  the  same 
time  the  current  temperature,  as  indicated  by  the  minimum  ther- 
mometer, should  be  recorded,  also  the  time  of  the  observation.  The 
current  temperature  is  principally  of  value  for  making  a  thermo- 
graph correction,  and  the  height  of  the  thermograph  pen  should 
herefore  be  recorded  at  the  same  time. 


RESEARCH  METHODS  IN  STUDY  OF  FOREST  ENVIRONMENT.         19 

When  readings  are  taken  in  the  morning,  if  there  is  no  thermo- 
graph record  by  which  the  time  of  the  maximum  and  minimum  may 
be  determined,  the  minimum  then  read  should  be  tabulated  on  the 
form  for  "Air  Temperature  Record,"  as  of  the  current  day,  and  the 
maximum  as  of  the  preceding  day.  If  readings  are  made  in  the 
afternoon,  both  maximum  and  minimum  should  be  credited  to  the 
current  day.  The  current  temperature  should,  of  course,  be  credited 
to  the  day  on  which  taken.  The  instrumental  corrections  should  be 
used  when  entering  the  data  in  the  field,  if  cards  therefor  have  been 
prepared,  the  card  being  tacked  in  a  conspicuous  place  in  the  instru- 
ment shelter. 

The  daily  range — purely  a  computed  quantity — in  degrees  and 
tenths,  should  be  the  difference  between  maximum  and  minimum 
temperatures  as  tabulated  for  any  calendar  day. 

Hourly  Temperatures. 

Where  a  thermograph  is  available,  the  instrument  should  be  set 
in  the  same  shelter  as  the  maximum  and  minimum  thermometers,  and 
hourly  temperatures  may  be  obtained  therefrom.3  Corrections  for 
the  thermograph  trace  should  always  be  obtained  from  the  readings 
of  the  maximum  and  minimum  thermometer,  as  thermograph  records 
are  liable  to  considerable  errors;  but  the  hours  to  which  these  correc- 
tions are  applied  may  well  be  a  matter  of  judgment  with  the  ob- 
server, depending  on  the  shape  of  the  temperature  curve.4  The  tabu- 
lation of  hourly  temperatures  when  obtained  will  require  the  special 
form,  " Hourly  (Air,  Soil,  or  Actinograph)  Temperatures.'  Certain 
data  therefrom  will  be  entered  on  the  "Air  Temperature  Record. ' 
For  example,  as  a  measure  of  conditions  affecting  growth  rate,  it  may 
be  desirable  to  know,  besides  the  mean: 

*  In  any  ordinary  comparison  of  the  temperatures  of  plant  habitats,  hourly  temperatures  are  not  likely 
to  be  used  except  to  explain  transient  phenomena.  However,  the  thermograph  is  an  exl  remely  valuable 
adjunct  in  determining  the  maximum,  minimum,  and  mean  temperatures,  not  only  helping  to  correct 
errors  of  observation  butmaking  possible  the  more  exact  determination  of  the  extremes  and  temperature 
ranges  for  any  period,  such  as  the  midnight-to-midnight  day,  which  is  the  unit  of  time  in  most  meteoro- 
logical computations.  . 

^  Various  rules  for  applying  corrections  to  thermograph  traces  are  used  by  different  students. 
obvious  that  errorsmay  exist  in  the  traces  from  two  distinct  causes:  (1)  When  the  range  of  oscillation  ol 
the  pen  is  too  great  or  too  small  the  thermograph  may  read  correctly  at  medium  temperatures  bul  be  high 
and  low  at  the  two  extremes;  (2)  even  if  the  pen  is  approximately  correct  in  its  possible  range  there  i 
due  both  to  the  lesser  sensitiveness  of  the  thermograph  as  compared  with  a  mercurial  thermomet  er  and  to  the 
friction  of  the  pen  upon  the  paper,  so  that  normally  the  pen  does  not  quite  reach  I  o  I  he  exl  remes  indicated 
by  the  thermometers.    In  the  first  case,  it  is  essential  that  the  error  be  distributed  some*  h  ding 

to  the  temperatures;  thus,  if  the  pen  read  correctly  at  a  temperature  of  45,  at  all  temperatures  ftbo> 
the  range  of  the  pen  being  too  great-there  would  be  a  minus  correction  for  the  trace,  and  al  all  tempera- 
tures below  45  there  would  be  a  plus  correction.    On  the  other  hand,  if  the  instrumenl  is ,  «q  ,erly  adjusted 
it  is  logical  to  apply  a  minus  correction  to  all  descending  portions  of  the  trace  and  a  plus  correel  ton  to  all 
ascending  portions,  the  amount  of  such  corrections  to  be  determined  from  the  corrections  at  .  he  minimum 
and  maximum,  respectively. 


20 


BULLETIN  1059,  U.   S.   DEPARTMENT  OF  AGRICULTURE. 


- 

o 
o 

- 

- 

p 

- 

- 

- 


> 

s- 

3 

"3 

"w 

a 

<s 

o 

> 

t-l 
CD 


O 


o 

St 

O 

e 


to 

•8 


>^ 
e3  to 

1 

HO 

o 

■  s 


to 

GO 


ft, 


5 


Z 
> 

— 

CO 

.a 

C 

Remarks. 

Frost  in 
morning.1 

(Indicate  by  X.) 

Snow  on 
ground. 

Number  of  hours  with— 

°> 

o      O 
CM  S2 

~  03 

- 

33-41°. 

32°  or 
below. 

Mean. 

1  lourly. 

o 

Max.  f  Min. 

M 

o 

Daily 
Range, 

0 

• 

Current. 

Temp. 

1 

O 

= 

Min. 

o 

Max. 

o 

■ 

r 
- 

- 

.— 1 

ti 

7- 

— 

1" 

to 

1  - 

DC 

~ 

c 

: 

S 

o 
e 

2 

n 

jk; 

— 

>  ~ 

ec 

t- 

r 

3 

-1 

a 
- 

E 

- 

RESEARCH  METHODS  IN  STUDY  OF  FOREST  ENVIRONMENT.        2 1 


; 

.       ,       .          CO 
...         >-i 
•     •     •        03 

:  :  :    is 
•  :  :  :    fc 

"oS 

.    !    .       en 
•    •    •       >. 

■  ■     •     •         o3 

:  :  :  :    -c 

■  ■     •     •       o 

:  :  :  :    £ 

■5 

i 

'• 

c 

-I  CI  M  f  >o  CC  t-  c 
■1NNNNNNC 

29 

30 

31 

Dec.mean 

6 

s 

O 
£-1 

< 

i 

! 

< 

i 

H 

H 

3 
5 

CO 

s- 
05 

o 

X 

03 


-O 

3d 

CD  to 


oo 

d  >> 

03  03 

£s 

d  o 
£  o 

03  g 

5£ 


co 

o 
o 

05 
(h 

(-1 

05 

05 


CO 

03 

-V 


SXj 


CD 

> 

05  o 


03 


a    t*s~ 


d 

o    . 

s  * 

05 

xj 
S 

3 


-3 

fl 


5*s 

-c 

<45 

B 


*2z 

d  03 
03   05 

a* 

v""'  CO 

-co  >> 


1  g  kco'0 

St.  C  u 
h  mo  1 


= 
03 


d*8 

O  c     - 


X 
>> 

X! 

T3 
CD 

03 
o 

•— i 

fl 

<o 

X> 
X5 


O 

H  O  ^  s 

sc2  • 
5  S3  -fl  X 

M    CO   K    g 


o 

o 


03 

C 


(h   — 

O   C 


a   fe 


o 
X! 

CO 


O 

<-* 
M 
fl 


05   O 


+3  o  — 


05  g 

_  ggC ^co 

^  c3  c3nH  r/(  ^ 

JlSgcx  £"t3 


■^  £~d--'°  g 
V-  fl  C  N  ^  g 
O   05   Oj   CD  ~   fc- 


£.  o  > 

d  d£  05  p 

03  03     .  •  • 

05   05   O  O  O 


•^    '3  §  S  Z  Z  Z 


XI 

O 


22 


BULLETIN   1059,   U.   S.   DEPARTMENT  OF   AGRICULTURE. 


so 

o 


.o 


13 

S 

3 

O 

?■. 

Os 

g 

o 

•ho 

<w 

rO 

5h 

O 

, 

« 

w 

<?«» 

w 

o 

« 

E> 

•»o 

H 

g 

"«j 

<w 

tf 

£h 

^ 

s 

- 

Sh 

s 

H 

00 

s  r 

H 

^  r-4 

<""N 

Ah 
< 

C3 

*~ 

W 

HO 

r*i 

Is* 

O 

13 

£ 

•  •, 

EH 

■v 
£ 

u 

o 

<! 

ho 

o 

3 

o 

r4J 

t-i 

t- 

t— ( 

e 

O 

g 

- 

7s- 

p— i 

o 

>H 

- 

?1 

ti 

o 

o 

HO 

C3 

HO 

CO 


1  "• 

H    ^_ — ■ 

0  c  x    . 
fer-      Eh 


d 

cj 
O 

CO 

Hourly  temperature  i — Degrees  Fahrenheit. 

a 

1—4 
i—l 

o 

>— 1 

a> 

CO 

r~ 

eo 

io 

>* 

CO 

e. 

r-H 

T— ( 

o 

i-H 

CT> 

00 

tH 

CO 

o 

•>»< 

CO 

<N 

T-H 

• 

>> 

ft 

— 

e 

-■'. 

— 

«; 

« 

r- 

X 

o- 

= 

— 

?J 

r- 

— 

>-. 

C 

i  - 

or 

- 

- 

-i 

s 

— 
-i 

■  - 
-< 

r- 

s 

RESEARCH  METHODS  IN  STUDY  OF  FOREST  ENVIRONMENT.         23 


Mi   ;  I 

P 

— ; — \ — 

III  II 

CO 

d 

>    •     ■          ',   \ 

03 

>s 

!     ■     i          '.   J 

CD 

►O 

:  ;  :      : 

a 

^3 

•  t  •      *  i 

o 
a 

<w 

:  :  :      :  1 

!    !    !        ! 

CO 

a, 

•    •    ■        i 

CD 

v 

.    .    .        .  | 

CD 

s~. 

■    •    i        i 

cd 

ftn 

!    ! '  !        ! 

T3 

■    •    ■ 

d 

CO 

i.i        • 

■    ■    >        • 

CD 

•*H 

U 

■    ■    •        • 

+J 

.    .            • 

d 

•    •    •        • 

CD 

•    ■    i        • 

«, 

. 

A 

a 

.    .    . 

03 

tn 

•    •    •        • 

be 

*    •    *        • 

O 

!    !    I        ! 

d 

«i-i 

+3 

o 

03 

.    .    . 

*    '    '        ' 

s-. 

O 

...        . 

«—» 

.    •    •        • 

CO 

.    .    . 

-^ 

.    .        . 

-d 

CD 

H 

T) 

M  if! 

:     : 

i 

III              > 

CO 

I           • 

d 

t      *      t             • 

03 

■        ■        I                I 

CO 

I        •        >               • 

-    0 

.      .      i            • 

!    '.    ; 

*d 

It!                      • 

d 

!    !    !        ! 

03 

... 

A 

:  :  :      : 

+3 

d 

CD 

d 

■  •  i      . 

■  .  *      . 

••H 

...      i 

CO 

•  .  »      ■ 

CD 

•l-< 

•  •  •      • 

i- 

.  .  . 

1    :     : 

d 

CD 

•» 

»— < 

•*H 

I..      » 

o 

.  .  <      ■ 

CO 

...      < 

■— 

...      • 

O 

.  ■  »      • 

«*-< 

.  •  .-     • 

CO 

.  •  >      « 

Xi 

...      • 

4J 

• 

d 

•  ■I      • 

CD 

.  .  • 

H 

*  •  ■ 

«3 

■  .  i 

CO 

.  .  • 

d 

03 

.  .  t 

CD 

;     ; 

B 

'.    '.    ! 

TJ 

■    •    * 

d 

!    '.    . 

03 

*    .    • 

CO 

.    .    i 

CD 

CD 

H 

'    , 

M 

.    .    . 

CD 

.    .    .   | 

d 

■>— i 

CO 

CD 

■l-< 

(.4 

-*J 

>    .    • 

d 

.    •    • 

CD 

•    .    . 

s- 

;     ;     ; 

■               >    S_       ■ 

03 

■  •  » 

.  O     • 

i- 

•           •  **■ -»     , 

O 

. 

•         !       ~~ 

fe 

■  •  i    * 

2       S  5-  ° 

o  c  <-i      J 

3     2  o-* 
3     So 

24  BULLETIN   1059,   U.   S.   DEPARTMENT   OF   AGRICULTURE. 

The  number  of  hours  with  temperatures  32°  or  below. 
The  number  of  hours  with  temperatures  33°  to  41°. 
The  number  of  hours  with  temperatures  42°  or  above. 

Frosts. 

Since  the  actual  freezing  of  foliage  may  have  some  very  definite 
effects  on  plant  functions  apart  from  the  temperature  effect,  the 
occurrence  of  frost  on  the  ground  in  the  morning  should  be  noted 
and  recorded  at  the  time  of  regular  observations.  This  notation  is 
especially  important  in  cases  where  air  temperatures  near  the 
ground  are  not  being  recorded.  If  the  latter  are  available,  records 
of  occurrences  of  30°  F.  or  below  may  be  taken  in  lieu  of  frost  ob- 
servations. This  record  should  also  be  tabulated  on  the  "Air 
Temperature  Record."  It  may  be  of  value  in  determining  at  least 
a  normal  growing  season  for  some  types  of  vegetation. 

Mean  Temperatures. 

It  is  generally  accepted  by  climatologists  that  the  mean  tempera- 
ture for  the  day  is  sufficiently  well  represented  by  the  sum  of  the 
maximum  and  minimum  divided  by  two.  This  is  probably  less 
satisfactory  in  the  forest,  however,  than  in  most  open  situations 
where  insolation  and  radiation  are  not  interfered  with.  Where  a 
thermograph  is  in  use,  a  nearer  approach  to  the  true  mean  may  be 
obtained  from  the  sum  of  the  hourly  temperatures  divided  by  24. 
Means  for  the  day  should  be  entered  to  the  nearest  tenth  degree 
Fahrenheit.  Some  of  the  problems  connected  with  computations 
of  means  are  described  by  Hartzell  (22). 

The  means  for  the  decades  and  the  whole  month,  in  all  tempera- 
ture columns,  should  be  obtained  and  entered  to  the  nearest  tenth 
degree.  The  following  computations  are  suggested  for  valuable  com- 
parisons of  growing  conditions.  They  should  be  made  and  entered 
at  the  foot  of  Form  5.  All  should  be  taken  from  the  maximum 
and  minimum  temperature  records,  since  it  is  likely  that  some  of  the 
stations  to  be  compared  will  have  no  hourly  records. 

Mean  temperature  for  days  with  snow  on  ground. 

Mean  temperature  for  days  with  no  snow  on  ground. 

Xumber  freezing  days  without  thawing  (maximum  below  32°). 

Number  freezing  days  with   thawing   (mean   32°   or  below,    but   maximum 
above  32°  F.). 

Xumber  cold  days  (mean  32.1°  to  41.0°). 

Number  cool  days  (mean  41.1°  to  50.0°). 

Xumber  moderate  days  (mean  50.1°  to  60°). 

Number  warm  days  (mean  60.1°  to  72.0°). 

Number  hot  days  (mean  72.1°  or  above). 


RESEARCH  METHODS  IN  STUDY  OF  FOREST  ENVIRONMENT.         25 

Annual  Summary 

The  annual  summary  of  air  temperatures  on  the  "Summary" 
form  should  be  a  tabulation  by  decades  and  months  of  the  means  or 
totals  obtained  from  the  "Air  Temperature  Record,"  with  the 
annual  mean  or  total,  as  the  case  may  be,  computed  therefrom. 
Usually  a  separate  "Summary"  form  will  be  used  for  each  datum 
to  be  summarized. 

In  addition,  as  a  part  of  the  annual  summary,  there  should  be 
worked  out  the  mean  or  total  for  each  datum  for  the  growing 
season.  The  limits  of  the  latter  may  be  determined,  as  indicated  by 
the  discussion  in  earlier  paragraphs.5  Whatever  the  criterion  as  to 
the  actual  length  of  the  growing  season,  it  should  be  considered  to 
begin  and  end  with  even  decades,  and  all  means  computed  for  the 
growing  season  should  be  the  sum  of  the  decade  means  divided  by 
the  number  of  decades. 


Form  10. 


[U.  S.  Forest  Service,  Physical  Survey.] 
SUMMARY. 


Type 


.;  station  No ;  datum 

height  or  depth 


Year. 


Dec- 
ade. 

Month. 

Mean 
annual. 

Jan. 

Feb. 

Mar. 

Apr. 

May. 

June. 

July. 

Aug. 

Sept 

Oct. 

Nov. 

Dec. 

Mean 
grow- 
ing 
sea- 
son. 


[Determined  by  comparing  annual  means  for  1  foot  and  4  feet.] 

Instruments. 

Approximate 
Thermometers  and  shelters:  range   of  prices. 

Mercurial  thermometer  (Weather  Bureau  pattern) $1.  25  to  $3.  00 

Maximum  thermometer  (Weather  Bureau  pattern) 2.  50  to    5. 00 

Minimum  thermometer  (Weather  Bureau  pattern) 1.  50  to    3.  00 

Maximum  and  minimum  thermometers  are  often  supplied  in 

pairs. 

Support  for  maximum  and  minimum  thermometers 2.  00  to    2.  50 

Instrument  shelter,  complete,  without  supports 20.  00  to  30.  00 

a  As  a  matter  of  fact  the  temperature  conditions  that  delimit  the  growth  of  plants,  and  especiaUy  of 
coniferous  trees  are  not  known,  and  to  attempt  to  fix  a  rule  for  determining  when  the  growing  season 
begins  and  ends  would,  at  this  stage,  be  extremely  arbitrary. 


26  BULLETIN   1059,   U.    S.    DEPARTMENT  OF   AGRICULTURE. 

Recording  instrum  ents: 

Thermograph,  complete,  with  a  year's  supply  of  blank  forms, 

pen  and  ink $70.  00 

(  ombined  air  and  soil  (or  water)  thermograph,  complete,  with 

bulb  and  connecting  tube  10  feet  long;  a  year's  supply  of 

blank  forms 105.  00 

Extra  length  tube  (above  10  feet)  for  above  instrument,  per  foot. .  .  50 

Thermograph,  short  range  of  temperature  (probably  duty-free 

prices) 32.  00 

Thermograph,  large  range  of  temperature  (probably  duty-free 

prices) 42.  00 

Recording  thermometers,  dial  type,  with  one  or  two  pens  and 

bulbs 75.  00  to  150.  00 

SOIL   TEMPERATURES. 

Soil  temperatures  are  probably  even  more  important  in  forest 
study,  especially  when  questions  of  initiation  and  distribution  are 
involved,  than  air  temperatures.  Opportunities  for  obtaining  data 
on  the  former  will  probably  be  more  restricted,  because  of  the  greater 
difficulty  and  expense  of  installing  satisfactory  apparatus.  They  are 
at  present  measured  at  very  few,  if  any,  Weather  Bureau  stations. 

It  should  be  strongly  emphasized  that  the  study  of  soil  tempera- 
tures is  in  a  primitive  stage,  and  that  the  devising  of  both  instru- 
ments and  methods  offers  great  opportunity  for  the  investigator, 
especially  in  the  search  for  the  exact,  controlling  conditions  of  the 
soil's  surface.  The  present  discussion  does  not  attempt  to  consider 
all  the  special  investigations  which  are  undoubtedly  needed,  but  con- 
fines itself  largely  to  routine  methods,  by  which  a  broader  survey  of 
soil  temperature  conditions  may  be  gained,  making  possible  regional 
and  site  comparisons  on  something  like  a  standard  basis. 

Purposes  to  be  Served. 

The  number  and,  to  a  considerable  extent,  the  method  of  soil  tem- 
perature observations  to  be  made,  will  depend  on  the  object,  Some 
of  the  purposes  to  be  served  may  be  summarized  as  follow  > : 

1.  Rather  general  comparisons  of  temperature  conditions  in  dif- 
ferent plant  formations  and  regions.  For  this  purpose,  soil  tem- 
peratures may  have  some  advantage  over  air  temperatures,  in  that 
the  former  reflect  to  a  considerable  degree  the  amount  of  insolation 
received  at  the  ground;  and  it  must  be  admitted  that  air  tempera- 
tures, without  radiation  measurements,  really  give  no  indications  of 
the  temperatures  experienced  by  the  plant.  For  this  broad  purpose, 
temperatures  at  a  depth  of  one  foot  are  perhaps  most  satisf  actor  v. 

2.  Very  careful  comparisons  of  the  extreme  temperatures  to  which 
plants  are  subjected  on  the  various  sites.  There  is  much  reason  to 
believe  that  maximum  temperatures  are  often  the  forbidding  fac- 
tor in  the  extension  of  the  range  of  any  given  species  and  that  they 


RESEARCH  METHODS  IN  STUDY  OF  FOREST  ENVIRONMENT.         27 

react  mainly  upon  the  very  young  seedling  at  the  ground  line,  where 
temperatures  are  usually  highest.  The  proper  investigation  of  this 
subject  certainly  demands  surface  measurements,  but  temperatures 
at  a  depth  of  a  foot  or  more  may  give  some  indication  of  the  sur- 
face condition.  The  actual  measurement  of  surface  conditions  pre- 
sents great  technical  difficulties,  which  will  be  pointed  out  later. 

3.  Determination  of  soil  freezing,  not  as  a  directly  operative  tem- 
perature condition,  but  in  relation  to  the  availability  of  soil  mois- 
ture. In  this  connection  it  must  be  borne  in  mind,  of  course,  that 
soil  moisture  may  become  essentially  nonavailable  at  a  temperature 
as  high  as  34°;  and,  again,  that  it  may  not  actually  freeze  until  a 
temperature  of  30°  or  lower  is  reached.  Soil  temperatures  for  this 
purpose  must  therefore  be  coordinated  with  some  data  on  the  soil 
itself  and  on  the  plants  involved.  It  is  obvious  that,  to  serve  the 
purpose,  frequent  observations  may  be  necessary.  Continuous  ther- 
mograph records  are  preferred  because,  while  most  forest  trees  are 
not  sensitive  to  freezing  for  short  periods,  if  at  all,  in  the  considera- 
tion of  moisture  even  a  short  period  of  relief  through  thawing  may 
mean  the  beginning  of  a  new  cycle  of  observations  on  the  effect  of 
drought.  In  the  study  of  soil  freezing,  the  surface  or  near-surface 
temperatures  are  perhaps  most  important,  but  it  is  not  entirely 
certain  that  mere  freezing  of  the  surface  soil  will  stop  the  movement 
of  water  through  the  main  root  and  stem.  It  is  the  part  of  caution, 
therefore,  to  examine  the  entire  root  zone,  and  it  may  perhaps  be 
necessary  to  know  the  conditions  of  the  tree  itself  as  regards  freez- 
ing at  a  point  near  the  ground  line. 

Problems. 

The  problems,  then,  to  which  soil  temperatures  are  related,  are 
even  more  numerous  than  those  concerning  air  temperatures  and  in- 
volve more  directly  the  relations  with  initiation,  habitat  extension, 
and  plant  succession,  rather  than  rates  of  growth.  Some  of  the  most 
evident  problems  may  be  listed  as  follows : 

1.  Optimum  temperature  of  the  soil  as  a  seed  bed  in  direct  effect 
on  rate  and  amount  of  germination. 

2.  Optimum  temperature  of  the  soil  in  stimulating  osmosis  in  the 
roots  and  hence  rate  of  growth. 

3.  Minimum  temperature  at  which  water  is  available  or  sutli- 
ciently  available  to  supply  transpiration. 

4.  Temperature  at  which  the  soil  freezes  and  cuts  off  the  plant 
entirely  from  water,  length  of  such  periods,  and  atmospheric  condi- 
tions conducive  to  transpiration  during  such  periods. 

5.  Maximum  temperatures  of  the  soil  or  soil-surface  which  may  be 
tolerated  without  injury  to  root  or  stem  of  the  young,  shallow-rooted, 
and  barkless  seedling. 


28  BULLETIN   1059,   U.   S.    DEPARTMENT   OF   AGRICULTURE. 

6.  Influence  of  air  temperatures  and  light  upon  soil  temperatures, 
especially  maxima,  with  different  kinds  of  soil  cover. 

7.  Correlation  between  soil  temperature  and  extremes  of  drought 
in  the  surface  soil.  It  should  be  noted  that  the  distinction  between 
drought  injury  and  heat  injury  to  young  plants  is  often  very  diffi- 
cult, as  is  shown  by  Hartley's  (44)  work.  Also,  that  the  soil  sur- 
face can  not  for  long  be  excessively  hot  without  becoming  arid. 

Time  of  Observations. 

The  daily  range  of  temperatures  at  the  surface  of  the  soil  may  be 
considerably  greater  than  in  the  air  above,  and  for  the  study  of  sur- 
face conditions  the  thermograph  is  essential.  The  time  of  observa- 
tion of  thermometers  used  to  check  this  instrument  should  be  a  time 
when  radiation  and  absorption  of  heat  in  the  surface  soil  arc  about 
equal,  or,  in  short,  in  the  early  morning.  At  no  other  time  will  a 
real  check  be  found  possible,  because  the  thermometer  and  thermo- 
graph are  not  equally  sensitive  to  changes  and  do  not  absorb  direct 
sunlight  equally  well. 

The  daily  range  of  soil  temperatures  at  a  depth  of  1  foot  or  more 
is  so  slight  that  it  is  unimportant,  except  in  its  bearing  on  the  ques- 
tion of  determining  the  mean  for  the  day.  The  latter  must  often 
be  obtained  from  a  single  daily  reading  of  soil  thermometers,  and 
must  be  based  on  a  knowledge  of  the  diurnal  oscillation  for  the 
particular  site.  The  daily  range  at  1  foot  will  seldom,  if  ever,  exceed 
5°  F.,  and  at  2  feet  is  far  less;  so  that,  at  greater  depths  than  1  foot, 
almost  any  time  of  the  day  is  suitable  for  obtaining  approximately 
a  mean  temperature.  The  time  of  observations  may  therefore  be 
made  to  accord  with  other  observations  without  any  serious  disad- 
vantages. 

The  point  of  this  discussion  is  that  it  is  not  satisfactory  merely 
to  compare  the  soil  temperatures  of  several  sites  for  a  certain  time 
of  the  day,  since  at  the  time  one  soil  may  be  cooler  than  the  mean 
temperature  for  the  day  and  another  above  the  mean. 

Daily  Mean  Soil  Temperatures. 

The  simplest  way  to  secure  a  proper  comparison  of  sites  in  respect 
to  mean  soil  temperatures  would,  of  course,  be  to  determine  the 
maximum  and  minimum  for  each  day  and  to  average  these,  as  is 
commonly  done  with  air  temperatures.  However,  as  will  be  pointed 
out  in  the  discussion  of  apparatus,  registering  thermometers  for 
this  purpose  have  not  been  satisfactorily  developed;  so  that,  at 
present,  dependence  for  a  complete  record  must  be  placed  on  one  of 
the  several  types  of  soil  thermograph,  supplemented  by  frequent 
readings  of  a  thermometer. 


KESEAECH  METHODS  IN  STUDY  OF  FOREST  ENVIRONMENT.         29 

Granting  that  a  full  and  satisfactory  record  of  soil  temperatures 
may  be  obtained  by  the  use  of  the  thermograph,  it  may  still,  because 
of  the  cost  of  this  instrument,  be  impossible  to  obtain  the  desired 
comparison  of  a  number  of  sites.  The  best  alternative  would  seem 
to  be  to  make  one  thermograph  serve  for  a  number  of  stations  by 
placing  it  successively  at  the  several  stations  until  the  nature  of  the 
diurnal  oscillation,  for  a  given  season,  has  been  worked  out  for  each 
station.  These  oscillations  will  depend  so  greatly  on  the  character 
of  the  insolation  that  a  curve  for  one  point  could  hardly  be  expected 
to  apply  at  any  other  point.  With  a  mean  daily  curve,  however,  a 
single  thermometer  reading  each  day  may  give  a  very  good  basis 
for  approximating  the  mean  soil  temperature  for  the  day.  If  this 
is  convenient,  the  reading  may  be  timed  to  accord  with  the  most 
probable  hour  for  the  mean  temperature  to  occur. 

With  hourly  soil  temperatures  for  a  period  of  a  week  at  any  season, 
tabulated  on  the  "  Hourly  (Air,  Soil,  or  Actinograph)  Tempera- 
tures" form,  the  mean  hourly  temperatures  may  be  computed, 
as  well  as  the  mean  for  all  of  the  days  concerned.  From  the  former 
may  be  obtained  a  correction  factor  for  any  hour,  which,  added  to 
the  reading  for  a  similar  observation  hour,  will  give  approximately 
the  mean  temperature. 

For  instance,  a  study  of  station  A-l  at  Wagon  Wheel  Gap,  Colo, 
(steep  northerly  exposure),  at  midsummer,  showed  that  the  daily 
oscillation  was  about  1.35°,  that  the  mean  temperature  was  ap- 
proached very  closely  at  7  a.  m.  or  7  p.  m.,  the  minimum  not  occur- 
ring until  2  p.  m.,  and  that  the  correction  for  a  9  a.  m.  reading  on 
6  days  varied  from  +0.10°  to  +0.50°,  with  a  mean  correction  of 
0.34.  Similarly  at  station  A-2  (south  exposure),  it  was  found  that 
the  approximate  mean  would  be  read  at  5  a.  m.  or  4  p.  m.,  that  the 
minimum  occurred  at  noon,  that  the  daily  oscillation  was  2.37°.  and 
that  a  9  a.  m.  reading  must  be  corrected  by  -0.88°  to  give  the  mean 
for  the  day.     Corrections  for  six  individual  days  varied  from  -  0.50° 

to  -1.35°. 

Moore  (11)  states  that  at  a  depth  of  3  feet  daily  oscillations  are 
not  felt.  It  is  believed  that  they  are,  as  a  rule,  too  small  even  at 
2  feet  to  warrant  consideration,  although  in  excessively  insolated 
soils  the  procedure  described  for  1-foot  temperatures  may  be  fol- 
lowed. .  . 

Another  method  which  suggests  itself  for  determining  the  probable 
variation  from  the  mean  of  any  daily  temperature  reading  at  a  fixed 
hour  is  to  compare  the  annual  mean  temperature  at  the  shallow 
depth  with  the  mean  for  4  feet  or  greater  depth  where,  it  may  be 
assumed,  the  daily  values  are  not  affected  by  regular  oscillations. 
For,  while  at  any  time  the  deeper  soil  may  be  cooler  or  warmer  than 
the  surface,  the  deeper  soil  always  evincing  a  definite  wfcen 


30 


BULLETIN   1059,   U.   S.   DEPARTMENT  OF   AGRICULTURE. 


changes  are  in  one  direction,  still  there  is  no  basis  for  assuming  that 
for  whole  years  there  can  be  any  essential  difference.  Therefore,  if 
for  example,  the  mean  annual  temperature  at  1  foot,  as  shown  by 
8  a.  m.  observations,  is  49°  and  the  corresponding  temperature  at 
4  feet  is  50°,  there  is  every  reason  to  believe  that  the  8  a.  m.  readings 
at  1  foot  give  values,  on  the  average,  1°  below  the  corresponding 
daily  means.  When  the  oscillations  are  greatest  at  midsummer,  this 
correction  would  be  too  small,  and  in  winter  it  would  be  too  great; 
but  its  use  should,  at  least,  bring  us  nearer  to  the  true  mean  tem- 
perature for  any  given  period. 

Table  1  indicates  the  correction  factors  thus  obtained  for  a  num- 
ber of  stations  and  sites,  with  sufficient  description  of  each  to  show 
why  the  morning  temperature  is  much  or  little  below  the  mean  for 
the  day.  Practically  all  of  these  records  were  obtained  from  ther- 
mometers in  iron  pipes,  which,  by  conduction,  tend  to  create  a 
greater  daily  range  of  temperatures  in  their  vicinity  than  occurs  in 
the  soil  naturally.  From  these  data  it  will  be  seen  that  a  small 
variation  from  the  mean  is  likely  to  be  secured  if  (1)  the  aspect  is 
easterly  so  that  the  site  receives  early  insolation,  or  (2)  if  the  ob- 
servation hour  is  relatively  late,  or  (3)  if  the  natural  daily  range 
is  small,  as  is  usually  the  case  with  heavy  cover  and  to  some  extent 
on  slopes  which  do  not  receive  vertical  rays.  Finally,  insolation  late 
in  the  day,  though  probably  causing  a  large  daily  range,  may  bring 
a  morning  observation  relatively  high  on  the  descending  curve. 
These  data  will  be  principally  valuable  in  indicating  that  ever}'  site 
must  be  studied  independently. 

Table  1. — Probable  error  in  mean  1-foot  soil  temperatures  obtained  through  singh  daily 

observations. 

[Determined  by  comparing  annual  means  for  1  foot  and  4  feet .] 


Site. 


Station. 


I  crate  southerly  slope  open. . .  ^ . 

Southwesterly  slope,  some  trees 

Northeast  slope,  steep,  heavy  cover. 

East  slope,  some  cover 

Canyon  bottom,  heavy  cover 

Northwest  slope 

North  slope,  no  cover 

N  orth  slope,  full  cover 

Flat,  lit  tie  cover... 

Do ....'.'.'.'.'.'".'.'.'.'. 

North  slope,  one-third  cover 

Do 

North  slope,  high  altitude,  no  cover 
Gentle  easterly  slope,  little  cover.... 

North  slope,  steep,  cover 

South  slope,  steep,  some  cover 

North  slope,  heavy  cover 


F-l... 
F-2... 
F-3... 
F-4... 
F-5... 
F-6... 
F-7-8. 
F-9... 
F-ll . . 
F-12.. 
F-14.. 
F-15.. 
F-16.. 
M-l... 
W-Al. 
W-A2. 
W-F. . 


Average 
depn 
of  1-foot 
tempera- 
ture. 


S  a.  m.,  strong  radiation. 
Insolation  late  in  day. 
Insolation  early,  range  small. 
Insolation  early. 
Insolation  late,  if  any. 

Do. 
9  a.m.,  some  insolation  early. 
'.»  a.  m.,  little  insolation. 
7 -8 a.  m.,  heavy  snow  blanket. 
s-9  a.  m.,  insolated  all  day. 
9  a.  m.,  little  insolation. 

Do. 
10-12  a.  m.,  radiation  inteo 

8  a.  m.,  early  insolation. 

9  a.  m..  small  daily  ran. 

a  daily  rangi 
11-12  a.  m'. 


solu^ptaime^Tm  Wlth  telethermoscoPe>  so  that  depression 


is  due  solely  to  normal  depression  of 


RESEAKCH  METHODS  IN  STUDY  OF  FOREST  ENVIRONMENT.         31 

Readings. 

All  readings  of  soil  thermometers  should  be  in  degrees  and  tenths. 
Where  suspended  soil  thermometers  are  used  the  readings  should 
be  made  with  the  greatest  possible  speed  after  removing  the  ther- 
mometer from  its  seat,  and  care  should  be  exercised  not  to  expose 
the  thermometer  bulb  to  the  sun,  even  though  it  be  encased.  Ther- 
mometers on  the  surface  of  the  soil  should  be  read,  if  possible, 
without  frequent  disturbance  of  their  contact  with  the  soil. 

With  standardized  thermometers  the  correction  may  best  be  ap- 
plied before  recording  the  reading;  but  if  centigrade  readings  must 
be  transposed  to  Fahrenheit,  or  the  reverse,  it  may  be  best  to  make 
the  instrumental  correction  and  the  transposition  in  the  office  simul- 
taneously. 

Tabulation. 

The  daily  observations  may  be  tabulated  on  the  "Soil  Tempera- 
tures'1 form.  Where  two  observations  are  made  in  one  day,  both 
should  be  entered  on  the  line  for  the  day,  with  the  mean  for  the  day 
computed  from  each,  and  the  average  of  the  two  computed  means  in 
a  separate  column.  Where  more  than  two  observations  are  made,  it 
will  be  best  to  enter  all  at  their  respective  hours  on  the  "Hourly 
(Air,  Soil,  or  Actinograph)  Temperatures"  form,  and  to  enter  on 
the  "Soil  Temperatures'1  form  the  mean  of  all  readings  without 
any  corrections,  provided  the  three  or  more  readings  are  distributed 
well  between  the  times  of  maximum  and  minimum  temperatures. 

Hourly  Soil  Temperatures. 

Whenever  the  soil  thermograph  is  used,  or  when  eye  observations 
are  made  at  frequent  intervals,  the  hourly  values  should  be  tabulated 
on  the  "Hourly  (Air,  Soil,  or  Actinograph)  Temperatures ';  form. 
With  the  complete  record,  the  means  by  hours  as  well  as  by  day- 
should  be  computed  for  any  period  covered. 

In  addition,  from  the  hourly  record  the  maximum,  minimum,  and 
mean  for  each  day  (midnight  to  midnight)  may  be  transferred  to  the 
"Soil  Temperatures"  form,  in  order  that  the  mean  may  be  compared 
with  that  obtained  from  thermometer  readings,  and  that  the  daily 
range  may  be  shown. 

The  application  of  corrections  to  the  soil  thermograph  trace  can 
not  follow  the  same  rules  as  are  used  with  air  thermographs,  because 
of  the  difficulty  of  making  corrections  of  the  maxima  and  minima  at 
the  time.  If  the  correction  of  the  soil  thermograph  trace  varies  con- 
siderably in  amount  from  day  to  day,  the  amount  of  the  correction 
at  any  hour  should  be  determined  by  its  position  with  respect  to  the 
preceding  and  following  correction  hours.  If  the  correction  at,  say, 
9  a.  m.  is  about  the  same  from  day  to  day,  one  correction  may  be 


32 


BULLETIN   1059,   U.   S.   DEPARTMENT   OE   AGRICULTURE. 


© 

> 


CO 

8 


© 


o 

CO 

s 
s 

fa 

CO 


CO 

- 
- 

« 

PL, 

- 


to 

S 

o 

g 


S 
o 

S 

o 


s 

J  Si 


I 


o 

■  — 
i*a 


o 

- 


03 
c 

i          . 

© 
C/3 

4-> 

© 

0) 

Daily 
range. 

03 

t 

t 

c 
« 

i 

3 

© 

.H  o 
En 

'. 

© 
u 

.° 

03 

s 

o 
< 

d 
S 

© 

.3  C 

X 
03 

© 

©  © 

e 

r 

> 

3 

Com- 
puted 
mean. s 

Temporal 

Cor. 

for 

time.3 

Ob- 
served 
temp. 

• 

0 

-- 
s 

73 

— 

© 

ft 

• 
-  -' 

>  tc 

<J  03 

Com- 
puted 
mean. 

Cor. 

for 

time. 

• 

Ob- 
served 
temp. 

Hour 
read. 

• 

Temporal  me 
surf. 

0  b- 
temp. 

Hour 

read. 

9 
P. 

r-^C 

1  CC 

"* 

'-' 

■c 

I- 

K 

O! 

03 

- 

a 

- 

-i  :r  —  ■- 

.:  i- 

-i 

C 
3 

~ 

- 
- 

~> 

-> 

-:, 

— 

i 

RESEARCH  METHODS  IN  STUDY  OF  FOREST  ENVIRONMENT. 


33 


j 

1 
1 
1 
1 

; 

. 

— 

: 

: 

- 
1  5 

- 

V 

>  i- 
1  y 

i 
) 

d 

03 

s 

d 

A 

d 
o 

a 

03 
+-> 
O 

H 

c 
c 

c 

a 

a 

is 

> 
i 

1 

I 
> 

2  +2  -a  s?„  a  2 
m  >>.o  <s  o  <u 

«  c3 


c3  a»  - 


- 


03  a> 
>  o 

r-f 

a)  d 


^T-5 


O+j  Q3 

03     S-l  T? 

boo 


O'S 


83 

°.  =£  "^  +a  w 

K*>03  u   O   m 


03  ■ 

a 
o 


03    03    CO 
>     r„     3 


03 


03 

o 


03 


H   03 
03  <j 


§2   03   P» 
03  fl  £  .3  ,Q 

§  -,   £  03 

*->  ej  - 

^(3    03    S3  - 

S  °gHg 

r-  *^  03  o  2 

2  §  ^  ~  O  as  -3 

"3.C   03M    gS   « 

-3  d  g  03 
d  O  O   _  g 
§  p  °-d  O 

—    03  "«  n3  ^H   -)-3        . 

g    «    C«    H  ^    C 


a     >> 

g'd'a 

■  03 

a 

o 


03 


~*   * 


33     CO    03r;-J 


3    1«H    tifl 
M  O  60m    c3  c3 

3*.s   gs 


>3 

*!3 

Si, 


+-> 

03 

O 

«- 

H 

+J 

o 

o 

«-H 

iH 

O 

03 

C3 

e^H 

3 

rji 

Ph 

' — ^  t 

0      ^-v 

°_  <T^=>.  g 

R  o  o  2  £ 

<M      •      •  tO   O 

o  +°+^  o 

&&&U 

---im  ^s_i-   ■ 

frozen 
cold  (3 
cool  (4 
warm 
hot  (6C 

o  o  o  o  o 

33    03   CO   02    02 

03    03    C/3    CO    03 

>>>>>>>>>> 

T3*  "^  T3  ^  'O 

dddcid 

2 

^ 

£ 

X 

iz 

10163— 22— Bull.  1059- 


34  BULLETIN  1059,  U.   S.   DEPARTMENT  OF   AGRICULTURE. 

applied  to  all  the  hours  of  the  day  which  it  represents.     This  impli< 
of  course,  that  the  range  of  the  instrument  has  been  adjusted  before 
it  is  placed  in  service. 

Summary  of  Soil  Temperatures. 

In  addition  to  the  means  by  decades  and  months,  the  "Soil  Tem- 
peratures "  form  may  be  shown  the  number  of  days,  for  each  depth  at 
which  readings  are  taken,  with  temperatures  below  :V2°  F.  (frozen); 
with  temperatures  32.1  to  41.0°  (cold);  with  temperatures  11.1  to 
50.0°  (cool);  with  temperatures  50.1  to  60.0°  (warm):  and  with 
temperatures  above  60°  (hot). 

Annual  Summaries  of  Soil  Temperatuki  s. 

The  ^Summary"  formnuvy  be  used  for  a  summary  <>f  one  or  several 
soil  temperature  conditions,  such  as  the  mean  temperatures  by 
decades,  months,  growing  seasons,  and  years,  or  the  number  «>f  days 
of  each  temperature  class  in  each  month.  In  the  case  of  surface 
temperatures,  the  mean  and  absolute  maxima  and  the  daily  pang 
are  doubtless  of  great  interest.  As  many  forms  a-  necessary  may  he 
used. 

It  may  be  found  that  a  given  soil  temperature  sufficiently  delimits 
growth  so  that  the  occurrence  of  such  a  temperature  marks  the  he- 
ginning  and  end  of  the  growing  season.  This  has  been  the  idea  in 
suggesting  a  division  of  temperature  computation-  at  M  I\  or  5°C, 
such  a  temperature  being  approximately  the  minimum  for  activity  of 
lower  forms  of  plant  life,  as  shown  by  numerou-  experiment-. 

Apparatus. 

The  most  simple  apparatus  for  measuring  soil  temperature-  i-  the 
encased  soil  thermometer,  having  a  stem  of  sufficienl  length  so  that 
the  mercury  appears  above  the  surface  of  the  ground  when  the  bulb 
is  at  the  desired  depth.  As  ordinarily  made,  however,  this  thermom- 
eter is  not  only  very  expensive  but  is  inadequately  protected  from 
exposure  to  the  elements  and  to  mechanical  forces.  For  this  reason 
it  is  not  desirable  for  permanent  stations,  but  will  probably  in  many 
cases  be  useful  where  observations  are  temporary  and  light  equip- 
ment is  desired. 

For  permanent  stations  the  most  serviceable  apparatus  that  has 
been  thoroughly  tried  is  an  ordinary  thermometer  suspended  by  ;i 
cord  in  a  1-inch  iron  pipe,  whose  lower  end  may  be  sealed  either  by 
a  cap  or  by  welding.  The  latter  is  preferable  where  the  pipe  must 
be  sunk  to  any -great  depth,  since  the  welded  pipe  may  he  formed 
as  a  wedge  and  may  be  driven  into  position  without  seriously  dis- 
turbing  the  soil.  The  pipe  should,  in  all  eases,  he  long  enough  to 
extend  well  above  the  ground  and  above  any  ordinary  >n<»\\  cover- 


1 


RESEARCH  METHODS  IN  STIDV  OF  FOREST  ENVIRONMENT.         35 


ing,  and  the  upper  end  should  he  capped,  the  suspending  cord  being 
attached  to  the  inside  of  the  cap.  A  welded  pipe  may  he  driven  in 
almost  any  soil  if  the  upper  cap  is  screwed  on  tightly,  and  a  mallet 
is  used  in  driving,  or  wood  is  placed  hetween  the  cap  and  hammer 
used.  An  iron  hammer  directly  applied  will  tear  the  cap  to  piec<  - 
in  a  few  blows. 

The  conductivity  of  an  iron  pipe  is  so  great  that  its  use  for  soil 
temperatures  at  a  depth  of  1  foot  or  less  introduces  serious  com- 
plications.    Wood  or  porcelain  tubes  are  therefore  necessarv. 

A  porcelain  wall  tube,  such  as  is  commonly  used  in  wiring  build- 
ings, may  ordinarily  be  obtained  in  lengths  up  to  one  foot  or  more 
at  electrical  supply  shops. 

For  a  relatively  permanent  installation  of  thermometers  at  a 
depth  of  about  a  foot,  wood  tubes  turned  and  bored  in  a  wood- 
working shop  are  very  satisfactory.  The  tube  should  have  some 
taper,  and  the  lower  end  should  be  pointed,  so  that  it  may  be  driven 
into  a  smaller  hole  that  has  been  made  with  a  bar.  A  wood  which 
does  not  split  readily  should  be  used.     When  completed,  the  tube 


Fig.  1.— Sectional  view  of  turned  wood  tube  for  soil  thermometers  at  a  depth  of  1  foot.  TchtluTmoscope 
(electric  resistance  thermometer)  with  one  bulb  and  recording  galvanometer  $245;  extra  bulbs,  each  $15; 
connecting  wire,  per  foot  about  SO. 10. 

and  its  plug  should  be  boiled  and  cooled  in  a  bath  of  creosote  and 
linseed  oil  to  prevent  swelling,  shrinking,  and  cracking.  The  top 
of  the  tube  may  be  turned  with  a  slope  outward,  and  the  plug  simi- 
larly turned,  so  that  rain  water  does  not  enter  readily.  A  tube  which 
has  proven  very  satisfactory  in  Forest  Service  work  is  shown   in 

Figure  1. 

A  satisfactory  tube  for  temporary  use  may  be  made  by  cutting  a 
piece  of  2  by  2  inch  lumber  14  inches  long,  boring  a  1-inch  hole 
through  from  end  to  end,  capping  the  lower  end  with  a  piece  oi 
tin,  and  cutting  a  plug  to  fit  in  the  opening  at  the  top.  Two  inches 
of  the  tube  should  be  left  above  ground.  It  is  hardly  feasible  to 
prepare  this  apparatus  in  greater  lengths;  in  fact,  for  depths  ol  2 
feet  or  more,  the  iron  pipe  is  to  be  preferred. 

In  order  to  obtain  reliable  readings  with  a  thermometer  which 
must  be  lifted  to  read,  it  is  necessary  that  the  bulb  ot  the  ther- 
mometer be  in  some  way  protected  from  immediate  contact  with  the 
air.     This  is  done  either  by  placing  it  in  a  cork,  by  wrapping  it  in 


36  BULLETIX    1059,   U.    S.    DEPARTMENT   OF   AGRICULTURE. 

tissue  paper,  by  sealing  to  the  thermometer  an  empty  cap  or  vial, 
or  by  sealing  on  a  vial  filled  with  alcohol.  Covering  the  bulb,  of 
course,  retards  the  movement  of  the  thermometer  which  changes  of 
soil  temperature,  but  this  is  unimportant  as  compared  with  the  sud- 
den changes  which  would  result  from  bringing  the  naked  ther- 
mometer up  into  the  air.  The  thermometer  to  be  used  in  these  tubes 
should  be  the  Weather  Bureau  ''mercurial  thermometer,"  and  may 
be  kept  attached  to  the  aluminum  frame,  which  affords  much  needed 
protection. 

The  use  of  registering  maximum  and  minimum  thermometers  in 
soil  temperature  work  is  not  very  satisfactory.  It  is  true  that  the 
standard  Weather  Bureau  types  of  these  instruments  may  be  used  on 
the  surface  of  the  soil  almost  as  well  as  in  the  air.  The  following 
precautions,  however,  should  be  observed: 

1.  To  bring  the  thermometer  into  close  contact  witli  the  soil,  ami 
to  avoid  unnecessary  conduction  the  metal  frame  should  be  discarded. 

2.  The  minimum  registering  thermometer  should  be  protected  from 
insulation  in  the  middle  of  the  day,  since  such  thermometers  ordi- 
narily will  not  bear  temperatures  in  excess  of  120°  F.  Also,  there  is 
some  tendency  to  distill  the  spirits  and  break  the  spirit  column  at 
high  temperatures. 

3.  The  thermometers  must  be  nearly  level. 

Maximum  and  minimum  thermometers  of  the  ordinary  type  are 
not  feasible  at  any  depth,  because  they  can  not  be  kept  level. 

A  maximum  thermometer  may,  however,  he  used  in  a  vertical 
position  at  any  depth,  provided  the  stricture  of  the  capillary  tube  is 
sufficiently  close  to  carry  the  weight  of  the  mercury  above  it.  This 
is  technically  almost  impossible  to  accomplish,  but  one  in  a  dozen 
maximum  thermometers  may  serve  the  purpose. 

To  use  the  registering  minimum  thermometer  at  any  depth,  it  is 
necessary  that  the  stem  be  bent  at  a  distance  from  the  bulb  approxi- 
mately equal  to  the  contemplated  depth,  and  that  the  scale  fall 
entirely  in  that  part  of  the  stem  above  the  bend,  which  is  to  be 
horizontal.  There  is,  of  course,  a  limit  beyond  which  this  form  of 
construction  can  not  be  safely  carried,  since  the  alcohol  in  the  stem, 
as  well  as  in  the  bulb,  reflects  temperature.  An  additional  difficulty 
is  in  the  distillation  that  has  been  mentioned,  but  this  may  be  largely 

• 

overcome  by  sufficiently  high  air  pressure  above  the  spirit  column. 

For  permanent  stations  the  use  of  the  telethermoscope  or  electric- 
resistance  thermometer  may,  in  some  cases,  be  advisable;  but  this 
apparatus  is  expensive  and  delicate,  can  not  be  installed  except  with 
considerable  disturbance  of  the  soil,  and  is  subject  to  serious  errors 
if,  for  example,  the  batteries  become  weak  or  the  galvanometer  i-  not 
perfectly  leveled.     Especially  where  great  precision  is  necessary,  as 


RESEARCH  METHODS  IN  STUDY  OF  FOREST  ENVIRONMENT.         37 

in  determining  the  character  and  influence  of  surface  temperatures, 
the  telethermoscope  offers  great  possibilities. 

The  soil  thermograph  is  desirable  for  continuous  records  of  soil 
temperature  which  can  hardly  be  obtained  with  any  other  instru- 
ment, and  particularly  for  measuring  the  extremes,  which  it  is  almosl 
impossible  to  obtain  with  registering  thermometers.  These  can  be 
had  only  after  careful  adjustment-of  the  range  of  the  pen.  or  by  fre- 
quent checking  against  a  thermometer,  at  high  and  low  temperatures, 
after  the  instrument  is  installed.  This  instrument  should  at  leasl 
be  employed  wherever  new  stations  are  being  established  and  until 
the  daily  curve  has  been  worked  out  for  each  season.  The  securing 
of  a  record  with  this  instrument  is  very  similar  in  its  routine  fea- 
tures to  the  work  with  air  thermographs.  There  is.  however,  one 
feature  of  the  soil  thermograph  which  deserves  special  consideration. 
This  is  the  tendency,  whenever  the  instrument  is  moved  and  the  con- 
necting tube  suffers  more  or  less  deformation,  for  the  whole  appa- 
ratus to  go  through  a  gradual  readjustment.  One  frequently  finds 
the  pen  steadily  ascending  or  descending  for  a  week  after  any  change. 
For  this  reason  it  does  not  appear  practicable  to  calibrate  or  adjust 
the  recording  apparatus  to  agree  with  an  accurate  thermometer  be- 
fore placing  the  instrument  and  its  bulb  in  their  final  positions.  The 
ideological  Society,  in  outlining  methods  for  a  systematic  soil  tem- 
perature survey,  however,  recommended  calibrating  soil  thermographs 
by  placing  the  bulb  in  a  pan  of  water  with  the  thermometer,  and  after 
placing  the  bulb  in  its  final  position  in  the  soil  trusting  completely 
to  the  accuracy  of  the  thermograph. 

It  is  believed,  in  view  of  what  has  been  said,  to  be  absolutely 
necessary  to  have  a  thermometer  so  placed  in  a  wooden  or  porcelain 
tube  that  its  bulb  is  at  the  same  level  and  pratically  in  contact  with 
the  bulb  of  the  thermograph,  and  to  obtain  frequent  comparisons  of 
the  thermograph  and  thermometer. 

Special  Suggestions  ox  Surface  Measurements. 

It  has  been  stated  that  the  extremely  high  temperature  attained  at 
the  surface  of  a  well-insolated  soil  seem  to  have  an  important  heal- 
ing on  the  initiation  of  plants,  and  that  technical  difficulties  make  the 
actual  measurement  of  this  surface  temperature  almost  impossible. 
Doubtless  this  could  be  accomplished  from  time  to  time  with  a  super- 
sensitive plate,  such  as  constitutes  a  part  of  the  leaf-temperature 
apparatus,  but  the  problem  of  recording  the  maximum  attained  in 
a  day  or  a  season  would  still  be  unsolved. 

It  "should  be  admitted,  therefore,  that  a  record  of  the  maximum 
temperature  at  the  soil's  surface  can  only  he  approximated  with 
present  equipment,  for  the  very  simple  reason  that  the  object  which 


38  BULLETIN   1059,   U.    S.    DEPARTMENT    OF   AGRICULTURE. 

is  to  indicate  the  temperature  can  not  be  expected  to  read  to  insola- 
tion quite  as  the  soil  does,  nor  to  be  exactly  in  temperature  equilib- 
rium with  the  soil. 

Since  the  technique  has  not  been  well  developed,  the  following 
suggestions  are  made  in  the  hope  of  obtaining  somewhat  comparable 
results  by  different  investigators. 

1.  The  bulb  of  the  (maximum-registering)  thermometer  or  the 
bull)  of  the  thermograph  should  be  exactly  half  buried,  the  object 
lying  in  a  horizontal  position.  The  lower  surface  may  then  be  a 
little  cooler  than  the  surface  of  the  soil,  but  the  exposed  surface  may 
be  a  little  warmer. 

2.  In  order  that  the  thermometer  or  thermograph  bulb  may  have 
absorptive  capacity  for  insolation  similar  to  that  of  the  soil  studies, 
the  exposed  surface  should  be  coated  with  linseed  oil,  and  while  this 
is  still  moist  enough  soil  should  be  sprinkled  upon  it  to  from  a  thin 
coating.  It  may  be  necessary  to  repeat  this  at  rather  frequent  in- 
tervals. 

3.  The  thermometer  or  thermograph  should  be  disturbed  as  little 
as  possible,  since  if  the  soil  about  it  is  kept  loose  it  will  not  be 
normally  moist  and  will  not  have  the  temperature  of  undisturbed 
soil.  A  maximum  thermometer  of  the  ordinary  type  must,  of  course, 
be  raised  for  setting,  so  that  for  frequent  comparisons  of  thermo- 
graph and  thermometer  the  ordinary  cylindrical-bull)  mercurial 
thermometer  may  be  most  satisfactory. 

4.  Provision  must  be  made  for  recording  temperatures  far  in 
excess  of  those  of  the  air  or  deeper  soil.  It  will  be  safest  to  allow 
for  an  excess  of  full  100°  F.  over  the  1-foot  soil  temperature  when- 
ever direct  insolation  is  received  during  several   hours  of  the  day. 

Instruments. 

For  mercurial  thermometers,  combined  air  and  soil  thermographs, 
and  recording  thermometers  (equally  adapted  to  air,  soil,  and  water 
measurements),  see  " Instruments"  listed  under  "Air  tempera- 
tures'': 

Special  soil  thermometers,  wood  encased,  with  stem  long  enough  to  be 
read  above  the  surface  of  the  ground,  for  depths  of  6  indies  to  3 

feet $0.  00  to  sio.  00 

Soil  (or  water)  thermograph,  with  connecting  tube.  pens.  ink.  forms. 

etc.  (bulb  is  about  1  inch  by  12  inches) 00 

Soil  thermograph 

Telethermoscope  (electric  resistance  thermometer  i  with  one  bull,  and 

nonrecording  galvanometer 95. 00 

Switch  for  0   thermocouples  (galvanometer    requires    about   3   dry 

cells) L6.00 


RESEARCH  METHODS  IN  STUDY  OF   FOREST  ENVIRONMENT.         39 

SOLAR    RADIATION— LIGHT. 

The  importance  of  light,  at  all  stages  in  the  development  of  tre< 
has  never  been  underestimated  by  foresters.  On  the  contrary,  re- 
viewing the  literature  of  forestry  at  the  present  time,  it  would  seem 
that  this  element  of  the  environment  has  been  emphasized,  by  some 
almost  to  the  exclusion  of  all  the  other  conditions.  It  was,  perhaps, 
only  natural  that  casual  observers  of  the  l'< >n>-t  should  mention 
this  factor  more  frequently  than  all  others,  because  the  presen 
or  absence  of  light  is  so  easily  detected.  It  must  now  he  admitted, 
however,  that  visible  light  does  not  tell  the  whole  story;  and,  fur- 
thermore, that  phenomena,  commonly  called  by  foresters  the  "sup- 
pression;:  of  trees,  which  have  often  been  credited  to  insufficient 
light,  may  be  and  probably  are  in  many  instances  caused  by  lack 
of  moisture. 

In  this  country  Zon  (77),  citing  experiments  of  Fricke,  was  one 
of  the  first  to  call  attention  to  the  relatively  great  importance  of 
factors  other  than  light.  It  may  be  that  his  suggestion  created  too 
strong  a  reaction,  that  there  has  been  too  much  of  a  tendency  on  the 
part  of  American  investigators  to  ignore  light  or  to  be  satisfied 
with  an  incomplete  study  of  its  ecological  relations.  It  is  believed, 
however,  that  this  is  only  apparently  the  case,  the  situation  being 
explained  by  the  large  amount  of  ecological  study  that  has  been  per- 
formed in  the  western  mountain  forests,  where  sunlight  is  not  defi- 
cient and  precipitation  or  soil  moisture  appears  usually  to  be  the 
more  vitally  controlling  factor. 

On  the  other  hand,  the  work  of  Burns  (56-59)  shows  substantia] 
progress  in  the  study  of  light.  Its  effects  on  growth  have  been 
directly  observed,  and  its  bearing  on  the  transpiration  rate  and 
water  requirements  of  young  trees  is,  at  least,  strongly  suggested. 
Pearson  (12),  Clements  (60),  and  many  others  have  made  observa- 
tions on  light  under  less  controlled  conditions.  The  principal  lesson 
to  be  learned  from  the  progress  to  date,  however,  is  that  light  can 
not  be  taken  independently  without  regard  for  other  conditions 
that  it  modifies  and  all  of  the  plant  functions  which  it  stimulati 
Nowhere,  perhaps,  is  a  better  illustration  found  of  the  danger  of 
one-sided  ecological  investigations  than  the  common  error  <>(  forest- 
ers in  ascribing  all  bad  effects  of  crowding  in  the  forest   to  lack  of 

lis;ht  • 

Concept  of  the  Functions  of  Radiant  Energy. 

The  solar  radiation  available  to  the  plant  not  only  supplements 
the  heat  available  by  conduction  from  the  air  but  is  vitally  necessarj 
to  the  chemical  activities  of  the  plant,  of  which  photosynthesis  is 
foremost  and  of  most  direct  interest  to  the  ecologist.  Ii  is  fairlj 
evident  that  sunlight  has  an  influence  on  the  temperature  oi  leaves 


40  BULLETIN   1059,   U.    S.    DEPARTMENT   OF   AGRICULTURE. 

and  other  plant  parts  of  which  we  obtain  only  a  partial  measure- 
meat  through  ordinary  air  temperatures.  That  this  is  an  important 
condition  affecting  distribution  of  eyery  species  is  evidenced  by  the 
fact  that  with  increase  of  both  altitude  and  latitude,  or,  in  short, 
with  decrease  of  air  temperature,  a  giyen  plant  seems  to  require 
more  light  for  its  development.  This  evidence  is  not  in  itself  con- 
clusive, because,  on  a  given  site,  more  light  obtained  by  wider 
spacing  will  usually  mean  more  moisture,  which  may  often  be  the 
controlling  factor.  Again,  in  a  given  locality,  the  species  which 
tli rive  best  in  air  of  low  temperature  always  seem  more  tolerant  of 
shade. 

Perhaps  it  is  best  to  analyze  the  situation  at  the  outset  according 
to  physical  principles  and  logic  rather  than  on  the  basis  of  ques- 
tionable evidence.  The  latter  has  been  mentioned  to  forewarn  the 
student  of  some  of  the  pitfalls  of  poorly  conceived  observational 
methods. 

The  radiant  energy  available  to  the  plant  may  consist  of  an  infinite 
variety  of  rays  or  wave  lengths,  from  the  most  subdued  heat  to  the 
ultra-violet  light.  The  effect  of  each  of  these  wave  Lengths  is 
entirely  dependent  upon  the  nature  of  the  absorbents  in  the  plant. 
Thus  the  organic  material  of  the  cell  walls  and  the  water  within  the 
cells  are  capable 'of  absorbing  readily  the  red  and  infra-red  or 
'heat;;  rays  of  the  solar  spectrum.  The  chloroplasts  show  an 
ability  to  absorb  visible  rays,  the  proportionate  absorption  of  the 
various  wave  lengths  varying  in  different  plants.  Of  the  absorp- 
tion of  ultra-violet  light  by  leaves  practically  nothing  is  known  as 
yet  on  account  of  the  difficulties  of  observation  in  this  end  of  the 
spectrum.  We  may,  however,  safely  assume  a  considerable  absorp- 
tion of  these  invisible  rays. 

There  is  practically  no  question  that  each  of  the  chemical  elements 
found  in  the  plastids  (or,  for  that  matter,  anywhere  in  the  leaf 
cells)  absorbs  the  kind  of  rays  which  it  would  absorb  under  any 
other  condition.  Thus  the  "selective  absorption'  by  different 
plants  may  be  mainly  the  resultant  of  different  amounts  and  pro- 
portions of  such  of  the  elements  as  iron,  sodium,  and  potassium. 

All  rays  which  are  absorbed  are  heating,  and  all  may  assist  in 
bringing  about  chemical  reactions,  of  which  the  first  in  importance 
to  the  plant  is  the  union  of  H20  and  C02  to  form  carbohydrate 
The  function  of  the  chlorophyll  and  of  the  chloroplasts  is  to  con- 
centrate sufficient  energy  at  a  given  point  to  effect  this  difficult  com- 
bination. The  kinds  of  rays  which  are  essential  to  photosynthesis 
therefore,  are  the  kinds  which  substances  in  the  chloroplasts  are 
capable  of  absorbing;  and,  as  has  been  said,  the  substances  may  vary 
according  to  the  kind  of  plant  and  according  to  the  solutes  which 
the  soil  is  capable  of  supplying. 


RESEARCH  METHODS  IN  STUDY  OF  FOREST  ENVIRONMENT.         41 

On  the  other  hand,  if  the  chloroplasts  find  themselves  in  a  medium 
of  cell  sap  which  is  cold,  it  is  perfectly  evident  that  the  energy  which 
has  been  concentrated  in  them  through  the  absorption  of  a  special 
assortment  of  rays  may  be  dissipated  to  the  surrounding  medium 
by  the  simple  process  of  conduction.  The  rate  of  conduction  will 
decrease  directly  as  the  temperature  of  the  cell  sup  approaches  that 
of  the  plastids.  It  is  thus  seen  that  both  radiant  energy  and  heat  of 
the  air,  which  may  serve  to  warm  the  leaf  as  a  whole,  do  have  an 
influence  on  photosynthesis;  and  that  for  a  given  intensity  of  sun- 
light there  must  be  a  leaf  temperature  below  which  photosynthesis 
can  not  be  effected,  because  of  the  dissipation  of  the  energy  in  the 
plastids.  This  leaf  temperature  will  depend  on  every  atmospheric 
condition,  including  the  air  temperature.  The  most  important  factor 
tending  to  keep  the  leaf  temperature  below  the  air  temperature  is 
the  use  of  any  available  heat  in  the  water-vaporizing  process  of 
transpiration.  This  consumes  a  very  large  proportion  of  all  the 
heat  obtainable  from  all  sources.  The  loss  of  water  and  consump- 
tion of  energy  is,  presumably,  to  be  looked  upon  as  an  unavoidable 
consequence  of  the  need  for  stomata  to  admit  carbon  dioxide. 

The  Nature  of  Sunlight. 

Biologists  must  enter  upon  the  measurement  of  radiant  energy, 
or  even  upon  a  discussion  of  the  subject,  with  the  greatest  hesitancy. 
realizing  (1)  that  the  physicists'  conception  of  energy  is,  at  this 
writing,  undergoing  a  change  almost  daily;  (2)  that  investigations 
of  the  solar  constant  and  of  sky  radiation  have  made  enormous 
strides  during  the  last  two  or  three  decades,  creating  a  vast  array 
of  equipment  none  of  which  is  of  proven  value,  and  leaving  the 
whole  situation  in  a  state  of  flux;  and  (3)  that  these  investigations 
have  shown' beyond  question  the  constantly  changing  quality  of  sun- 
light, due  both  to  variations  in  the  sun  itself  and  to  absorption  in  the 
earth's  atmosphere.  Realizing  these  things,  it  must  be  admitted 
that  the  past  investigations  of  light  in  connection  with  forestry  and 
other  biological  subjects  are,  practically  without  exception,  obsolete 
and  of  no  assistance  in  looking  into  the  problems  of  the  future. 

It  can  not  be  attempted  in  this  discussion  to  predict  the  line  of 
endeavor  for  future  investigators  in  light,  Plainly  it  is  a  problem 
for  specialists  only.  A  few  of  the  most  fundamental  facts  or  prin- 
ciples which,  it  seems,  must  govern  the  method  of  attack  at  this  tune 
may,  however,  be  pointed  out, 

1.  As  to  the  character  of  sunlight,  probably  the  most  important 
point  to  be  borne  in  mind  is  that  it  is  an  extremely  variable  quantity, 
both  as  regards  its  whole  energy  and  its  constitution  oi  various  wave 
lengths.  Setting  aside  for  the  present  the  fact  that  the  emanations 
from  the  sun  vary  periodically  in  total  intensity  and  also  in  wave 


42  BULLETIN   105!),    U.    S.    DEPARTMENT    OF    AGRICULTURE. 

length,  it  is  necessary  to  consider  the  constantly  changing  absorption 
by  the  earth's  atmosphere.  According  to  Very  (75),  who  cites  Lang- 
Ley  (Professional  Papers  of  the  Signal  Service,  No.  15),  there  are 
"two  different  kinds  of  selective  depletion  which  the  solar  ray-  suffer 
in  traversing  the  earth's  atmosphere.  One  kind  is  greatest  for  the 
rays  of  shorter  wave  length,  and  diminishes  by  perfectly  regular 
gradations  as  one  passes  toward  the  longer  waves  of  the  infra-red. 
Its  cause  may  be  referred  to  selective  reflection  or  diffraction  of  the 
shorter  ether-waves  by  particles  of  excessive  minuteness.  The  other 
kind  of  absorption  produces  irregular  gaps  or  depressions  in  the 
spectral  energy  curve,  which  begin  at  the  red  end  of  the  visible 
spectrum  and  grow  in  magnitude  and  frequency  as  the  wave  length 
increases.  Researches  by  Abney  and  Festing.  and  by  other  investi- 
gators, have  traced  the  majority  of  these  depressions  to  the  action  of 
aqueous  vapor."  In  the  extreme  infra-red  there  is  shown  to  be 
almost  total  absorption  from  this  source. 

The  light,  principally  of  the  shorter  wave  lengths,  which  is  dif- 
fused by  minute  particles  in  the  atmosphere  is  not  entirely  lost,  but 
may  be  measured  as  skylight,  probably  of  greater  wave  length  than 
the  original  direct  rays.  The  infra-red  rays  which  are  so  greatly 
absorbed  by  the  vapor  of  the  atmosphere  merely  heat  the  upper 
atmosphere,  and  to  this  extent,  of  course,  are  losl  as  solar  radiation. 

2.  Looking  at  the  matter  from  another  viewpoint,  and  accounting 
for  the  rather  regular  daily  change  in  sunlight  intensity  at  a  given 
point  on  the  earth's  surface,  Kimball,  (63)  after  showing  the  greater 
intensity  of  all  wave  lengths  at  midday  when  the  light  passes  through 
minimum  thickness  of  atmosphere,  makes  interesting  comparisons 
of  the  total  and  luminous  radiation  under  various  circumstances' 
Radiation  from  an  overcast  sky  is  slightly  richer,  and  radiation  from 
a  clear  sky  markedly  richer,  in  luminous  rays  than  is  direct  sunlight. 
Direct  sunlight  decreases  in  luminous  richness  as  the  sun  approaches 
the  horizon. 

3.  These  few  facts  point  to  the  uselessnc-  of  photometric  methods, 
depending  on  the  chemical  action  of  rays  of  rather  limited  wave 
length,  to  measure  the  total  radiation  or  any  part  of  the  radiation 
other  than  the  few  wave  lengths  which  may  be  involved  in  the  par- 
ticular reaction.  Thus,  for  example,  even  if  it  were  assumed  that 
silver  chloride  was  decomposed  in  proportion  to  the  intensity  of  a 
given  section  of  the  spectrum,  a  certain  reaction  with  silver  chloride 
might  be  secured  with  other  wave  lengths  varying  through  a  very 
wide  range. 

Again  quoting  Very  (75),  it  is  seen  that  photochemical  procesa 
are  very  complex  and  hazardous  as  a  measure  of  energy: 

While  luminous  effects  may  be  regarded  as  dependent  on  a  certain  photochemical 
action  upon  the  retina,  not  all  photochemical  procesaes  an-  equally  definite  and  m<  I 


RESEARCH  METHODS  IN  STUDY  OF  FOREST  ENVIRONMENT.         43 

urable.  As  M.  Radau  (72)  says:  "The  red  rays  and  the  yellow  rays  in  certain  ca 
continue  the  work  commenced  by  the  violet  rays,  and  in  others  undo  what  the  Lasl 
have  accomplished.  Thus,  chloride  of  silver,  slightly  impressed  by  the  violel  ra; 
is  then  blackened  under  the  action  of  all  of  the  visible  rays;  and  guaiacum,  turned 
blue  by  the  violet  rays,  is  bleached  by  the  more  luminous  rays,  h  follows  thai  the 
chemical  action  of  light  is,  in  general,  very  complex,  and  that  it  can  be  used  for 
measuring  the  energy  of  solar  rays  only  with  much  circumspection." 

The  inevitable  conclusion  is  that  direct  photochemical  methods  can 
not  be  made  to  solve  the  problems  of  ecology,  but  this  does  qoI  elimi- 
nate spectrophotochemical  measurements,  which  may,  in  fact,  give 
the  best  possible  criteria  as  to  the  variations  in  the  differenl  spectral 
regions  and  the  effect  of  such  variations  on  plants. 

4.  The  fourth  point  to  be  considered  in  approaching  the  study 
of  possible  methods  for  radiation  measurements  is  the  difficulty  of 
securing  complete  absorption  of  sunlight.  While  lampblack  is  popu- 
larly conceived  to  absorb  rays  of  all  wave  lengths  and  to  transform 
them  into-measureable  heat,  recent  investigations  have  proved  that 
this  is  only  approximately  true,  and  have  shown  the  existence  of 
an  infra-red  spectrum  of  extreme  wave  length  to  which  Lampblack 
is  partially  transparent.  Fortunately,  this  region  is  relatively  unim- 
portant as  a  source  of  energy  and  may  be,  for  biological  purposes, 
almost  wholly  unimportant. 

A  greater  source  of  error  than  that  arising  from  the  failure  of 
lampblack  to  absorb  the  radiation  is  undoubtedly  the  loss,  as  beat 
radiation,  and  by  conduction  and  convection,  before  the  heal  can  be 
properly  measured.  It  must,  of  course,  be  borne  in  mind  that  tem- 
perature is  not  a  measure  of  heat,  and  that  the  indications  of  a  ther- 
mometer can  not  be  used  except  as  the  radiation  rate  of  the  thermom- 
eter itself  has  been  thoroughly  studied. 

With  this  conception  of  the  nature  of  sunlight  and  the  difficulties 
in  the  way  of  its  proper  measurement,  it  is  perfectly  evidenl  that 
the  primitive  methods  that  have  been  employed  in  measuring  lighj 
in  the  forest  do  not  give  satisfactory  results.  Two  distinct  but  sup- 
plemental lines  of  attack  suggest  themselves  as  being  profitable: 

1.  The  growing  of  trees  under  controlled  conditions  of  light,  using 
both  artificial  lights  of  known  composition  and  monochromatic  and 
other  screens  which  will  transmit  to  the  plants  only  certain  wave 
lengths,  as  suggested  in  the  work  which  has  been  started  by  Mac- 
Dougal  (68):  The  physiological  action  of  each  wave  length  must,  "1 
course,  be  studied.  Through  such  study  it  is  hoped  that  the  require- 
ments for  light  may  be  determined,  any  actual  deficiency  in  sun- 
light which  exists  in  the  forest  recognized,  and  the  effective  supply 

measured. 

2.  In  studying  either  the  light  conditions  as  they  exist  id  the 
forest  or  the  effective  supply  suggested  by  the  preceding  line  oi 


44  BULLETIN    1059,    U.    S.    DEPARTMENT    OF   AGRICULTURE. 

investigation  it  is  obviously  necessary  to  use  spectroscopic  methods. 
Since  the  growth  of  a  tree  requires  many  years,  and  even  complete 
suppression  in  the  densest  forest  is  seldom  accomplished  in  less  than 
two  or  three  vears,  it  is  evident  that  in  the  forest  minute  examination 
of  every  variation  in  sunlight  is  unnecessary.  An  examination  cov- 
ering the  entire  period  of  the  activity  which  is  being  studied  must, 
however,  he  obtained.  The  nearly  ideal  and  still  practicable  arrange- 
ment would  seem  to  be  provision  for  continuous  observation  of  the 
total  energy  available  as  radiation  throughout  the  period  of  plant 
activity,  with  sufficiently  frequent  spectroscopic  observation  of  the 
composition  of  this  energy  to  establish  not  only  an  average  quality 
analysis  for  the  whole  period  but  also  to  show  the  variations  which 
occur  from  season  to  season  and  }Tear  to  year  and  their  relations  to 
the  functioning  of  the  plant. 

Unfortunately,  spectrum  analysis  by  present  common  methods  does 
not  permit  an  examination  of  either  the  ultra-violet  or  infra-red 
spectra.  For  this  reason  it  has  been  suggested  that  all  spectrum 
analyses  might  be  better  conducted  by  means  of  energy  measure- 
ments (e.  g.  thermal  effect)  than  by  optical  comparisons.  This  is  an 
almost  unexplored  field  and  presents  infinite  possibilities  for  the  in- 
vestigator who  will  devise  a  satisfactory  method  of  measuring  the 
energy  of  all  parts  of  the  spectrum  under  both  laboratory  and  field 
conditions. 

Horizontal  and  Vertical  Exposures. 

It  is  perhaps  well  to  point  out  at  this  stage  that,  particularly  in 
forest  studies,  light  measurements  of  whatever  kind  may  be  on  two 
distinct  bases.  In  forestry  the  growth  of  an  individual  tree  is  rarely 
spoken  of,  or  even  if  it  is,  no  practical  significance  is  attached  to  it. 
because  the  individual  can  rarely  be  separated  from  the  influence  of 
other  individuals.  Forest  growth,  in  any  practical  sense,  must  be 
growth  (volume  increment)  per  unit  of  land  area.  Similarly,  if  an 
attempt  is  made  to  find  a  relationship  between  growth  and  available 
light,  it  is  certain  that  the  energy  must  be  expressed  in  terms  of  a  unit 
of  area  inclined  at  the  same  slope  as  the  ground.  The  total  energy 
available  to  the  crowns  of  trees  on  a  northerly  exposure  of  given 
gradient,  for  example,  can  not  be  more  than  that  which  would  be  inci- 
dent upon  a  plane  of  exactly  the  same  aspect  and  gradient.  Land 
areas,  however,  are  always  measured  in  terms  of  their  horizontal 
projections.  It  therefore  follows  that  the  measurements  must  be  re- 
duced to  horizontal  areas,  and  the  simplest  means  for  reduction  is  to 
expose  a  given  area  horizontally  for  the  original  determinations. 

Determinations  of  total  energy  available  for  growth,  however, 
will  rarely  be  made  in  ecological  studies,  which  are  much  more  likely 


RESEARCH  METHODS  IN  STUDY  OF  FOREST  ENVIRONMENT.  I  5 

to  be  concerned  with  questions  affecting  the  survival  of  the  individual 
plant  or  tree.  The  individual  tree  of  any  age  must  be  thought  of  as 
a  spherical,  conical,  or  cylindrical  mass  of  irregular  surface,  such  thai 
a  pencil  of  rays  will  affect  equal  absorbing  surfaces,  almost  regard- 
less of  its  angle  of  approach.  It  is  therefore  logical  that,  in  the  study 
of  individual  trees,  all  determinations  of  light  intensity  and  quality 
should  take  as  the  unit  a  pencil  of  rays  of  given  cross  section.  Tin- 
full  cross  section  is  to  be  obtained  by  exposing  the  absorbing  surface 
of  given  area  normal  to  the  axis  of  the  pencil. 

In  the  following  discussion  exposures  for  this  purpose  and  of  tin- 
nature  will  be  implied,  unless  light  quantities  affecting  stand  growth 
are  specifically  mentioned. 

Total  Radiation  on  the  Site. 

To  determine  the  quantity  of  radiant  energy  which  is  available 
for  plants  or  trees  on  any  particular  site  in  relation  to  the  growth  of 
the  whole  stand,  obviously  the  quantity  should  be  pleasured  at  a 
point  where  it  has  not  been  intercepted  or  diminished  in  intensity. 
As  previously  pointed  out,  this  will  be  above  the  crown-  or  m  an 
opening  of  exposure  similar  to  the  plane  of  the  forest  canopy.  Alter 
having  determined  the  total  amount  of  energy  available,  the  amount 
actually  utilized,  if  desired,  might  be  measured  as  the  difference  be- 
tween the  total  and  that  which  is  available  below  the  canopy.  In 
any  event,  the  intensity  of  solar  radiation  may  be  expressed  in  heat 
units,  or  calories  per  square  centimeter  of  horizontal  area. 

Insolation  Under  Canopies. 

The  measurement  of  insolation  or  sunlight  intensity  under  canopies 
may  be  for  two  distinct  purposes:  To  determine  the  amount  of 
energy  which  has  escaped  the  tree  crowns  above  and  thereform  to 
deduce  the  amount  utilized  by  them;  and  to  determine  the  amount 
which  is  available  for  undergrowth,  either  in  the  form  of  subordi- 
nate species  or  reproduction.  The  first  measurement,  which  is  not 
concerned  with  the  tolerance  of  the  species,  but  rather  with  the  com- 
pleteness of  the  canopy,  the  completeness  of  utilization,  and  the 
rate  of  growth  of  the  stand,  should  obviously  be  closely  connected 
with  measurements  of  total  radiation  on  the  site.  Since  the  plan  oi 
such  measurements  has  been  explained,  this  subject  may  be  dismissed 
and  the  attention  turned  to  those  problems  winch  are  concerned 
wholly  with  the  question  of  tolerance,  or  the  question  ol  the  relative 
requirements  of  the  various  species  for  light,  especially  in  connec- 
tion with  survival  in  their  earliest  stages. 


46  BULLETIN   1059,   U.    S.    DEPARTMENT    OF   AGRICULTURE. 

Light  Measurements  in  Relation  to  Minimum  Requirements. 

The  tolerance  of  trees  to  shade  may  be  determined  in  any  one  of 
four  ways: 

1.  By  preparing  empirical  scales  of  tolerance,  based  on  experience 
and  long-continued  observation  of  the  relative  shade-enduring  quali- 
ties of  various  species  when  growing  together.  This  method  is  obvi- 
ously very  crude,  and  may  be  very  misleading,  since  such  a  condi- 
tion as  soil  moisture  may  determine,  as  directly  as  does  light,  the 
relative  positions  of  the  species  in  any  particular  stand.  The  per- 
sistence of  individual  branches,  or  rate  of  pruning;  the  maximum 
density  of  stands  composed  mainly  of  any  particular  species:  the 
ability  of  reproduction  to  thrive  in  shade;  all  these  things  may  be 
considered  in  preparing  empirical  scales. 

2.  A  second  method  of  determining  the  tolerance  of  trees  is  by 
study  of  the  structure  of  the  leaves.  Having  determined  the  normal 
relations  of  the  tissues  of  protective  and  assimilative  characters, 
leaves  may  be  subjected  to  different  degrees  of  shading.  Those  which 
adapt  themselves  most  completely  to  a  variety  of  light  condition- 
are  naturally  those  which  will  survive  best  if  placed  under  trying 
conditions  as  regards  lack  of  light.  This  method,  however  scientifi- 
cally it  is  executed,  can  not  give  us  absolute  comparisons,  since  the 
structures  of  leaves  are  so  variable  even  under  the  same  conditions 
that  the  exact  degree  of  change  of  structure  can  not  he  determined. 
In  other  words,  this  may  give  indications,  but  not  comparable  statis- 
tical data. 

3.  A  third  method  of  determining  tolerance  is  the  experimental 
method,  which  must,  of  course,  be  executed  in  the  laboratory,  where 
all  other  conditions,  as  well  as  the  supply  of  light,  may  be  con- 
trolled. The  primary  object  is  the  determination  of  the  minimum 
amounts  of  light  which  will  sustain  life  of  the  several  specie-  under 
consideration  when  all  other  conditions  (especially  heat  and  soil 
moisture)  are  nearly  optimum.  It  will  be  fairly  apparent,  however, 
that  high  temperatures  may  reduce  the  light  requirement,  and 
low  soil  moisture  may  increase  it;  and,  since  variations  in  all  of  the 
other  conditions  will  be  encountered  in  the  field,  it  is  very  desirable 
that  any  experimental  test  should  be  so  conducted  that  the  influence 
of  these  other  conditions  on  tolerance  may  be  at  least  accurately 
gauged,  if  not  directly  measured. 

It  is  believed  that  the  best  results  will  be  secured  if  each  species 
to  be  tested  is  grown  under  a  variety  of  light  conditions,  approaching 
both  the  optimum  and  the  minimum,  and  if  the  tests  are  so  conducted 
that  the  physiological  effects  of  each  light  intensity  may  be  expressed 
finally  in  terms  of  growth,  or  weight  accretion,  rather  than  if  depend- 
ence is  placed  solely  or  largely  on  observations  of  fatal  effects  when 


RESEARCH  METHODS  IN  STUDY  OF   FOREST  ENVIRONMENT.         47 

the  minimum  light  has  been  exceeded.  For  example,  if  seedlings  of  a 
given  species  are  grown  with  20,  40,  60,  80,  and  LOO  per  cenl  of  the 
full  available  light,  other  conditions  being  equal,  and  if  the  greatest 
accretion  is  put  on  by  those  having  60  and  80  per  cent,  while  tho$ 
having  only  20  per  cent  barely  exist,  and  some  of  their  number  suc- 
cumb, it  will  be  fairly  evident  that  the  optimum  is  between  60  and  80 
per  cent  and  the  minimum  slightly  below  20  per  cenl  for  the  given 
conditions  of  heat  and  moisture.  Both  points  may  be  found  quite 
closely  enough  by  curving  the  growth  data.  Similarly,  in  other 
temperature  and  moisture  series  different  optima  and  minima  of 
light  may  be  found,  and  the  absolute  optimum  combination  may  be 
very  nearly  arrived  at. 

On  account  of  the  difficulty  of  duplicating  any  set  of  conditions 
at  different  periods,  it  is  extremely  desirable  that  the  nunc  important 
species  whose  relative  tolerance  it  is  desired  to  know  should  all  be 
treated  during  the  same  period,  and  also  that  an  arrangement  should 
be  effected  which  will  make  possible  different  combination-  of  light 
with  moisture  and  temperature. 

The  following  plan  for  such  experimental  determination  of  toler- 
ance, while  merely  suggestive,  may  assist  in  initiating  stum'  work 
along  this  very  important  line.  The  arrangements  suggested  should 
accommodate  about  four  species.  It  would,  perhaps,  be  well  to  run 
an  initial  test  with  rather  gross  differences  in  the  light  quant  it  i 
as  suggested  above,  and  to  repeat  at  a  later  date  when  the  knowledj 
obtained  will  permit  more  minute  examination  of  the  critical  points: 

Construct  a  solarium  about  5^  by  8  feet,  with  its  long  axis  Iving 
east  and  west,  its  floor  and  glass  roof  having  possibly  a  gentle  -l<>p<- 
to  the  south.  The  depth  from  glass  to  floor  need  not  r\. •,.,<!  18 
inches.  Divide  this  into  three  equal  parts  by  means  of  glass  parti- 
tions running  north  and  south.  If  two  layers  of  glass  are  used 
throughout,  having  dead  air  between  them,  the  purposes  will  be  more 
completely  fulfilled  without  affecting  light  quantities  appreciably 
more  than  would  the  single  layer  of  glass.  Let  the  higher  north 
wall  serve  as  entrance  to  the  compartments,  being  closed  by  a  door 
whose  inner  surface  has  very  poor  reflecting  powers. 

For  each  of  these  compartments  10  pans,  each  a  foot  square  and  <» 
inches  or  a  foot  deep,  will  be  required.  These  may  be  made  ol  gal- 
vanized iron  with  drainage  openings  in  the  bottom.  Into  each  pan 
put  a  measured  quantity  of  soil,  sufficient  to  hi  it  to  within  .,  inches 
of  the  top.  The  pan  and  dry  soil  weight  both  having  been  deter- 
mined, the  amount  of  water  necessary  to  maintain  a  given  moisture 
percentage  in  the  soil  may  easily  be  computed  and  this,  added  to 
the  gross  dry  weight,  will  give  the  weight  which  the  pan  should  sho* 

after  each  watering.  .       ,    ,  i 

Each  pan  may  now  be  sown  with  sufficient  seeds  oi  the  several 
species  involved  to  produce  a  good  stand  on  the  area  o  1  square 
foot.     Possibly  100  seeds  of  each  species  should  he  used  in  ea<  Ik  tn< 


4:8  BULLETIN    1059,   U.    S.    DEPARTMENT    OF   AGRICULTURE. 

seedlings  later  being  thinned  to  uniform  density.  Unless  it  is  desired 
to  determine  the  effect  of  light  on  germination,  this  process  should 
be  concluded  for  all  pans,  under  uniform  conditions,  before  they  are 
placed  in  the  solarium.  The  main  operations  may  be  started  just 
as  soon  as  the  seedlings  are  established.  Otherwise  great  care  must 
be  used  to  develop  the  seedlings  similarly  in  all  pans. 

Having  reached  the  proper  stage,  place  10  pans  in  each  of  the  three 
compartments;  say,  in  two  north-and-south  rows.  The  three  com- 
partments are  to  be  maintained  at  different  temperatures;  say.  at 
mean  temperatures  of  50°,  60°,  and  70°  F.,  with  more  or  less  diurnal 
oscillation  in  each.  In  each  compartment  one  row  of  pans  is  to  be 
given  sufficient  water  to  maintain  its  soil  moisture  at,  say.  twice  the 
wilting  coefficient,  while  the  other  row  will  be  maintained  at  four 
times  the  wilting  coefficient.  The  condition  of  the  pans  may  at  any 
time  be  determined  by  weighing  and  the  water  supply  regulated 
accordingly.  In  any  row  of  five  pans  five  different  light  intensities 
may  be  maintained.  One  pan  in  each  row  should  doubtless  be 
allowed  full  sunlight,  another  should  be  cut  20  per  cent,  a  third  40  per 
cent,  etc.  The  amounts  may  be  governed  by  previously  gained 
empirical  knowledge  of  the  requirements,  so  that  the  full  range  of 
light  values  will  not  have  to  be  covered  in  any  case.  'Die  shading 
of  each  pan  separately  may  be  arranged  by  using  covers  of  punched 
screen,  in  which  the  areas  of  the  openings  correspond  to  the  proportion 
or  full  light  which  it  is  desired  to  admit.  It  should  be  borne  in 
mind  at  the  outset  that  the  glass  of  the  solarium  considerably  reduces 
the  light  intensity,  particularly  in  the  infra-red  rays.  The  quality 
of  this  light  should  be  compared  with  that  of  direct  sunlight,  and 
means  should  also  be  devised  for  measuring  the  light  intensity  under 
each  screen. 

The  proper  heating  of  the  various  compartments  will  prove  the 
most  serious  obstacle  in  most  cases.  This,  of  course,  will  have  to  he 
accomplished  by  artificial  means  and  should  be  done  by  introducing 
warm  air  into  the  compartments  from  an  outside  source  in  such  a 
manner  as  to  maintain  the  desired  air  temperatures  without  directly 
heating  the  pans.  The  air  thermometer  and  thermostat  should  be 
suspended  at  a  mean  elevation  and  protected  from  insolation. 

The  positions  of  pans  in  each  compartment  should  be  frequently 
changed  so  that  none  will  profit  more  than  others  by  Localized  heat 
and  light  optima,  which  are  certain  to  exist. 

The  final  effect  of  light  and  also  the  effect  of  other  factors  with 
light  is  to  be  determined  by  the  accretion  of  dry  matter  in  the  seed- 
lings of  each  pan  and  species.  In  order  that  tins  may  be  expressed 
in  net  quantities  for  the  time  of  treatment,  some  of  the  seedlings 
weeded  out  at  the  beginning  of  the  test  should  be  dried  and  weighed. 

It  is  evident  that  this  general  plan  might  be  followed  in  tests  to 
show  the  effect  of  different  kinds  of  light  on  growth,  using  mono- 
chromatic screens  as  covers  for  the  pans  instead  of  the  punched  me- 
tallic screens,  or  supplying  different  compartments  of  a  dark  chamber 
with  various  kinds  of  artificial  light. 

4.  The  fourth  method  of  determining  tolerance  or  light  require- 
ments is  similar  to  that  just  described,  but  depends  on  the  measure- 
ment of  light  intensities  as  they  are  encountered  in  the  field,  and  the 


RESEARCH  METHODS  IX  STUD}    OE  FOREST  ENVIRONMENT.         49 

correlation  of  such  measurements  with  observations  on  the  condition, 
rate  of  growth,  etc.,  of  the  trees  existing  under  the  measured  condi- 
tions. The  great  advantages  of  this  method  are  that  a  great  variety 
of  light  conditions  may  be  obtained  and  maintained  with  little  ex- 
pense or  trouble  and  that  growth  and  health  of  the  subjects  may  be 
studied  through  long  periods  and  under  natural  conditions.  One 
disadvantage  is  that  a  variety  of  light  conditions  is  uecessaril)  accom- 
panied by  a  variety  in  the  measure  of  other  conditions  the  effect  of 
which  may  be  confused  with  the  effects  of  light,  and  aeither  of  the 
two  sets  of  effects  can  be  exactly  measured  and  balanced  againsl  each 
other,  nor,  most  of  all,  can  they  be  controlled.  A  further  disadvan- 
tage consists  in  variation  of  the  amount  of  shading  at  any  given  point 
with  different  hours  of  the  day  and  seasons  of  the  year,  necessitating 
long-continued  observations  to  obtain  any  expressive  results.  Thi 
disadvantages,  however,  will  loom  up  less  formidably  when  we  under- 
stand better  what  part  of  the  radiant  energy  is  really  effectn  As 
has  been  pointed  out,  the  field  method  must  go  hand  in  hand  with 
laboratory  studies. 

Apparatus  and  Methods  for  Radiant  Energy  Measurements 

Although  most  of  the  methods  of  light  measurement  used  by  forest 
investigators  have  been  described  as  now  obsolete,  it  is  impossible,  of 
course,  to  throw  away  all  that  has  been  gained  through  experiem 
with  different  types  of  instruments.  Quite  apart  from  forest  inves- 
tigations, there  is  available  a  vast  amount  of  researeh  in  the  study 
of  light  per  se  which,  however  incomplete  and  changing  thi-  study 
may  be,  represents  the  starting  point  for  any  new  work  undertaken. 
It  is  therefore  considered  expedient  to  bring  together  a  list  of  the 
methods  and  instruments  which  have  been  used,  not  in  any  degr< 
of  historical  completeness,  but  rather  to  show  the  several  line-  of 
study  and  their  possibilities  as  briefly  as  possible. 

1.  In  the  radiometer,  which  is  commonly  seen  in  jewelers'  \\  Lndows, 
the  energy  of  light  is  transformed  into  work.  This  instrument  has 
no  practical  value,  however,  because  the  work  is  perforin,.!  inef- 
ficiently and  probably  does  not  vary  in  proportion  to  the  energy 

received. 

2.  The  thermopyle  represents  the  first  attempt  to  transform  radi- 
ant enersv  into  electrical  current.     This  is  accomplished  by  allowim. 
the  light 'to  fall  upon  the  junction  of  two  wires  of  differed  metalf 
The  opposing  ends  of  the  two  wires  are  also  joined,  forming  a  com- 
plete circuit.     The  amount  of  current  generated  a. ssing  around 

this  circuit  is  measured  at  any  point  in  the  circuit  by  means  ol  a 
galvanometer.     The  radiomicrometer,  for  measuring  the  heat  iron 
stars,  is  an  extremely  delicate  adaptation  of  the  principle  o(   the 

thermopyle. 

1  ( »i  rq_92— Bull.  1 059 ! 


50  BULLETIN   1059,   U.    S.    DEPARTMENT   OF   AGBICULTUBE. 

3.  The  bolometer  developed  by  Langley  (67),  who  was  a  pioneer 
in  investigations  of  the  sun's  energy,  employs  a  blackened  platinum 
strip  for  the  absorption  of  the  rays,  the  thermal  effed  on  this  strip 
being  measured  by  its  change  in  electrical  resistance. 

4.  The  later  developments  along  this  line,  described  as  pyrhdi- 
ometers  are  known,  respectively,  as  the  Angstrom  (69),  Callendar 
(64),  Marvin  (65),  and  Smithsonian  Institution  Standard  (50).  The 
first  three  are  constructed  on  the  principle  of  electric  resistance  ther- 
mometers, while  the  Smithsonian  utilizes  a  mercurial  thermometer. 
The  Callendar,  it  is  understood,  is  distinguished  by  its  automatic 
arrangements  for  constantly  recording  the  difference  in  resist  .nice 
between  the  absorbing  and  nonabsorbing  plates  or  "grids." 

The  technical  differences  between  the  several  types  of  instruments 
are  so  involved  that  a  discussion  of  them  can  not  be  undertaken  here. 
They  have  to  do  largely  with  questions  of  efficiency  in  absorption 
and  measurement  of  the  energy.  In  fact,  the  student  of  ecology 
who  plans  to  use  any  such  instruments  as  these  will  be  compelled 
to  make  a  most  thorough  study  of  the  subject.  Recent  year-  have 
seen  so  much  attention  given  to  it  by  physicists  and  meteorologists 
that,  it  may  be  said,  the  measurement  of  solar  radiation  is  in  a  >tate 
of  flux.  Bigelow  (53)  has  recently  questioned  the  adequacy  of  any 
measurements  made  with  pyrheliometers,  declaring  them  useless,  al 
least  for  the  determination  of  the  solar  constant.  It  will  therefore 
be  the  part  of  wisdom  for  biologists  to  stand  aside  until  the  physi- 
cists have  reached  a  more  stable  basis. 

In  all  of  these  instruments  the  auxiliary  apparatus  required  is 
considerable,  except  possibly  with  the  Smithsonian.  This  is  natu- 
rally a  deterrent  to  their  use  in  the  field,  although  the  difficulties 
may  always  be  overcome  when  we  are  convinced  of  the  usefulness 
of  the  results.  An  instrument  utilizing  a  mercurial  thermometer 
recommends  itself  for  simplicity;  yet,  in  view  of  the  frequent 
changes  in  the  light  intensity  at  any  single  point  in  the  forest,  the 
equipment  for  continuous  recording  is  not  more  than  is  needed  for 
satisfactory  results. 

5.  The  thermometric  sunshine  recorder  (70),  with  electrical  regis- 
tering apparatus,  is  the  equipment  used  at  many  Weather  Bureau 
stations  for  registering  the  duration  of  sunlight.  This  instrument 
is  extremely  simple  in  design  and  operation,  involving  only  the 
movement  of  a  column  of  mercury  through  a  tube  connecting  a  black- 
ened and  a  transparent  bulb  of  an  air  thermometer.  When  the 
mercury  reaches  a  certain  height,  as  the  result  of  air  pressure  in 
the  black  bulb,  it  completes  a  circuit  with  the  two  platinum  wires  cm- 
bedded  in  the  walls  of  the  tube,  and  the  current  passing  over  this 
circuit  operates  the  registering  mechanism. 


RESEARCH  METHODS  IN  STUDY  OF  FOREST  EXYIROX.M  I.M  .  .",  ] 

The  apparatus  is  adjusted  by  increasing  or  decreasing  the  amount 
of  mercury  in  the  tube  and  by  bringing  the  tube  closer  to  or  farther 
from  a  vertical  position,  so  that  the  mercury  first  reaches  the  platinum 
wires  when  the  disk  of  the  sun  is  visible  through  the  clouds.  An\ 
addition  to  the  light  intensity  above  this  approximate  standard  do 
not,  therefore,  alter  the  nature  of  the  record.  While  the  method  is 
thus  seen  to  be  extremely  crude,  the  record  showing  only  the  presence 
or  absence  of  light  of  a  rather  low  intensity,  still  it  can  hardly  he 
questioned  that  such  a  record  of  sunlight  duration  is  of  very  greal 
value  in  comparing  the  solar  climate  of  different  regions,  and  possibly 
also  in  obtaining  a  measure  of  the  direct  light  under  canopio ;  that  is, 
of  the  approximate  degree  of  shading.  There  appears  t<>  be  an 
untried  value  in  such  records  through  arbitrary  rating  of  the  recorded 
"sunshine"  according  to  the  elevation  of  the  sun,  and  with  allowance 
also  for  atmospheric  humidity. 

One  objectionable  feature  of  the  instrument  is  the  amount  of  time 
required  to  warm  it  in  the  morning  to  the  point  where  it  first  records. 
In  fact,  it  is  by  no  means  free  from  effect  of  the  air  tempera  tin-'  and 
must  be  adjusted  to  the  seasons. 

In  the  lack  of  a  better  measure  of  sunlight  values,  it  seems  well 
worth  while  to  have  this  sunshine  record  in  forest  studies.  The  form 
for  "Daily  and  Hourly  Sunshine  Duration ';  has  been  provided  for 
the  tabulations  of  a  month. 

6.  The  solar  thermograph  or  mechanical  differential  telethermo- 
graph  devised  by  Briggs  (54)  in  the  biophysical  laboratory  of  the 
Bureau  of  Plant  Industry  is  in  principle  the  same  as  some  of  the 
soil  thermographs;  but  it  is  a  duplex  instrument,  in  which  the  tem- 
perature of  one  of  the  bulbs  tends  to  compensate  that  of  a  second. 
One  of  the  bulbs  may  be  blackened  and  spherical,  with  a  short  tube, 
so  that  the  bulb  is  rather  easily  held  just  above  the  case  of  the  in- 
strument, while  the  second  bulb  may  be  kept  in  the  shade. 

This  arrangement  permits  the  recording  of  the  excess  of  tempera- 
ture attaineoT by  the  bulb  in  sunlight,  limited  by  the  natural  radia- 
tion and  by  conduction,  which  will  increase  as  the  air  movement  in- 
creases. The  reduction  of  air  movement  to  practically  zero,  or  the 
elimination  of  conduction  almost  entirely  by  the  use  of  an  evacuated 
glass  case,  would  make  possible  the  calibration  of  such  an  instrument 
so  that  the  temperature  difference  between  the  bulb  and  the 
rounding  air  might  be  directly  converted  into  rate  of  heat  absorp- 
tion by  the  bulb. 

As   a  matter  of  fact,  an  ordinary  air  and  soil  thermograph  has 
been  used  by  Bates  (105)  with  a  fair  degree  of  satisfaction  to  show 
the  variations  in  sun  heat  from  day  to  day,  the  disadvantage  oi 
regular  equipment  being  in  the  variable  surface  exposed  to  the  sun 
at  different  hours  by  a  cylindrical  bulb. 


52 


BULLETIN   1059,   U.    S.    DEPARTMENT   OF   AGRICULTURE. 


Z 

c 

-H 

««* 

PS 
P 

P. 

P 

Z 


CO 

Z 

P 

CO 

PS 
P 

o 
a 

p 
z 
< 

p 
i— i 

< 
p 


a 

•  o 


CO 

c 

3 
X 


C5 


o 


fc 


Kl 


s 


o 

8 

o 

o 

o 

•  ~^ 

<3 


<3<c 

■s 

e 

«0 


<» 

I 


a 

PS 


Ph 


o  to 

CO 

O 


C3£2  03 


OPS 


00 


ft 

0 

■—i 

rt 

o 
s- 

o 

u 
o 


-* 


CO 


03 

60 

C 

Pi 
CD 

H 

3 

O 

Xi 

o 


CN 


00 


o 


to 

H 

o 

XI 


CD 

t-i 
O 

to 

CB 

3 


3 
o 

— * 
03 


3 

rH 

3 

CO 


- 


— 

03 

9 


a 

= 


OJ 
C3 
P 


-^MMTfif.  (OSX05" 


ri  re  —  •  -  sc  i  -    i 

-i 


RESEARCH  METHODS  IN  STUDY  OF  FOREST  ENVIRON  KENT. 


CO 

S 
a 


03 


O 


a 
c 


pi 

03 

co 

a 
>> 

*i 
a 
o 


>.- 

«  be 

-c^a 

o3« 

CO  o 

*S 

■s 

CO      • 

3-i4 

3£ 

C.Q 

3« 

x  <_* 

«~  co 

C-1 

CO  ^3 

3  co 

r-i    CO 

■a-0 

a  >> 

c,  o3 

sa 

y.  O 

(Zl   CO 

ftp 

5>s 

*-,    *i 

iC 

©  <d 

3fl 

'e 

C    t, 

§3 

5> 

C 

A. 

-a  >» 

^ 

+j^^ 

fcto 

C  o 

3  2 

ft& 

as 

O   CO 

°.£ 

to 

r-i    CO 

CO   O 

gTfi 

s  » 

w  ft 

^S 

,3  w> 

MS 

3-r| 
O  p 

Si  +J 

-2 

H a 

£« 

s-a 

3^ 

%V, 

CO    «> 

3  > 

.-1    S-. 

2  ft 

3-° 

■^  o 

+^    _ 

C3« 

CO 

3  3 

£    © 

co,C 

X3  P 

03  s 

CJ'-3 

co  03 

> 

•c> 

CO 

3  co 

a 

Lis 

CO 

tS'co 

O 

O  co 

ft 

CO   ft 

CO 

Si 

03  £2 

a 

O 

■(-> 

03 

.  3  s 

> 

ons 
act 
giv 

CO 
CO 

+3  ofo 

O 

u 

03 

Si    S-.-1 

3 

6J3 
CO 
d 

ht  co 
obse 
shou 

CO 

M>>>co 

CO 

rl,Q  h 

CO 

'g-3  s 

^ 

a-a  s^ 

S  B  a  o3 

sg-aa 

-«   CJ 

P'O  o 

© 

,0 

<i  ©,-. 

O 

-i-s 

6^S 

CO 

a 

.-3  6CO 

£2.5 

-  c  a 

CO 

~ 

o 

tl 

3 

o 

54  BULLETIN   1059,    U.    S.    DEPARTMENT   OF   AGRICULTURE. 

There  is  hardly  any  question  that  an  instrument  of  this  kind  may 
be  made  to  serve  the  present  practical  requirements  of  forest  studies 
for  a  measure  of  radiation  intensity.  It  would  be  very  desirable  to 
have  the  necessary  protection  from  air  currents  supplied,  without 
intercepting  some  of  the  rays  by  a  layer  of  glass. 

7.  Photochemical  photometers  .—The  objections  which  have  been 
raised  to  the  use  of  chemical  reactions  as  a  measure  of  the  sunlight 
intensity  can  not  be  overcome.  In  addition  to  the  fact  that  the  other 
rays  may  not  vary  from  time  to  time  in  the  same  proportion  as  the 
chemically  active  rays  measured,  it  is  somewhat  questionable  whether 
the  result  secured,  as  in  the  coloration  of  a  photographic  paper,  is 
proportionate  to  the  product  of  the  light  intensity  and  the  time. 

In  forest  studies  the  photochemical  method  seems  to  serve  one 
purpose  fairly  well,  that  being  to  obtain  a  measure  of  the  density  of 
the  canopy.  It  is  then  assumed  that  the  amount  of  light  reaching 
the  ground  is  to  the  total  sunlight  as  the  area  of  openings  in  the 
crowns  is  to  the  whole  area;  or,  in  other  words,  that  the  lighi  coming 
through  these  openings  is  unaltered  in  its  passage.  While  technically 
there  is  also  light  below  the  crowns,  which  has  been  transmitted 
through  the  leaves,  and  the  photographic  paper  may  be  sensitive  to 
the  rays  in  this  class,  it  probably  does  not  introduce  any  serious 
error,  considering  the  purposes  for  which  such  measurements  should 
be  used. 

This  crown-density  determination  should  always  be  made  by  mov- 
ing the  photometer  through  as  great  a  space  as  possible  during  the 
few  seconds  of  exposure. 

The  Bunsen-Roscoe  (55)  unit  of  actinic  light  is  the  light  required 
to  produce  on  a  standard  paper  a  shade  equivalent  to  that  produced 
by  the  mixture  of  1  part  lampblack  and  1,000  parts  pure  white  zinc 
oxide.  The  details  of  the  preparation  of  this  "normal  shade'  and 
the  " normal  paper"  are  given  by  Zon  and  Graves  (78)  or  may  be 
obtained  from  the  original  citation  given  above.  The  object  in  men- 
tioning it  here  is  simply  to  show  that  it  is  possible  to  carry  on  photo- 
metric observations  on  a  fixed  standard. 

Likewise,  photographic-supply  manufacturers  have  prepared  a 
standard  shade  and  a  standard  paper  for  estimating  light  intensi- 
ties. One  of  the  best  known  of  the  " exposure  meters'1  is  extremely 
simple  in  operation.  In  one  opening  of  a  small  disk  containing  the 
standard  paper  is  exposed  the  standard  color,  a  permanent  shade.  In 
a  corresponding  opening  may  be  seen  a  fresh  area  of  the  paper.  It 
is  only  necessary  to  expose  this  to  the  light  until  it  attains  the  stand- 
ard shade  noting  the  time  required,  to  have  a  very  close  basis  for 
computing  the  light  intensity.  It  seems  that  this  simple  contrivance 
may  serve  the  purpose  of  ecologists  quite  as  well  as  more  elaborate 
apparatus  of  the  same  type. 


RESEARCH  METHODS  IN  STUDY  OF  FOREST  ENVIRONMENT.         55 

Weisner   (76)   used  an  instrument  almost  as  simple  as   this,   bis 
standard  shade  and  fresh  paper  being  set  in  a  groove  in  a  block 
wood  and  appearing  in  openings  of  a  layer  of  opaque  paper. 

Clements  (6)  devised  a  photometer  in  which  a  narrow  strip  of 
"solio"  or  other  sensitive  paper  may  be  held,  having  sufficient  area 
for  25  exposures.  This  strip  is  placed  on  the  periphery  of  one  metal- 
lic ring,  which  fits  snugly  inside  another.  In  the  outer  ring  there  is 
an  opening  one-fourth  of  an  inch  square,  covered  b\  a  9lide  winch 
is  drawn  back  to  make  the  exposure.  In  using  this  instrumenl  the 
colors  obtained  on  exposure  are  not  directly  compared  with  a  stand- 
ard color.  Rather,  it  is  customarv  to  make  a  scale  of  shades  with 
each  set  of  observations,  consisting  of,  say,  10  exposures  in  full  light, 
of  1,  2,  3,  etc.,  seconds.  In  the  later  exposures,  then,  it  is  only  acces- 
sary to  keep  within  the  limits  of  the  "scale, "and  the  time  may  be 
varied  to  secure  the  desired  shade.  If,  for  example,  a  24-second  ex- 
posure gives  a  shade  corresponding  to  7  seconds  on  the  scale  for  full 
light,  the  relative  value  of  the  suppressed  light  is  7/2  1  or  29  per  cent. 

The  photochemical-photometer  method  is  not  satisfactory  for  any 
expression  of  the  light  in  absolute  terms,  or  for  comparing  quantities 
in  one  day  or  season  with  another  day  or  season,  or  for  comparing 
different  localities.  Of  course,  all  exposures  might  be  compared  to 
some  standard  shade,  but  the  operation  is  needlessly  circuitous 
and  is  made  the  more  difficult  by  the  perishable  nature  of  the  record, 
the  need  for  examining  it  in  dim  lamplight,  etc.  It  is  therefore  be- 
lieved that,  while  this  method  has  some  value,  a  similar  efforl  ex- 
pended in  detennining  absolute  light  quantities  will  be  much  more 

profitable. 

In  addition  to  the  above  there  are  instruments  of  the  same  prin- 
ciple by  which  more  or  less  continuous  records  may  be  secured.  In 
one  such  instrument,  used  by  the  Weather  Bureau  a  number  of  years 
ago,  the  light  entering  through  a  very  small  opening  made  its  i  re- 
pression as  a  band  across  the  sheet  of  photographic  paper  on  which  it 
fell  at  successive  moments.     (See  Clements  (6),  p.  51. 

8.  It  is  probable  that  several  hundred  kinds  of  comparison  /> 
tometers  have  been  devised,  all  depending  on  an  ocular  comparison 
of  the  sunlight  to  be  studied  with  a  light  of  known  luminous  powers 
It  is  evident  that  such  instruments  deal  only  with  the  Luminous  raj  *, 
and  while  they  rely  upon  the  accuracy  of  the  eve.  the  method  cer- 
tainly has  advantages  over  the  photochemical  method  in  dealing  w  i 
that  part  of  the  spectrum  which  is  least  affected  by  changes  in  atmos 
pheric  absorption.     It  is  not  to  be  supposed,  however,  that  the  lumi- 
nous rays  control  plant  activities. 


56  BULLETIN   1059,    U.    S.   DEPARTMENT   OF   AGRICULTURE. 

One  of  the  simplest  types  of  comparison  photometers  is  the  smoked- 
glass  type  used  by  Wagner  and  credited  to  him  by  Zon  and  (da- 
(78),  although  undoubtedly  invented  much  before  his  time.  The 
smoked  glass  is  in  the  form  of  a  wedge  which  is  inserted  between 
the  eye  and  the  source  of  light,  until  the  thickness  attained  by 
moving  it  one  way  or  the  other  is  just  sufficient  to  cause  complete 
absorption  of  the  rays.  The  wedge  is  calibrated  along  it-  entire 
length,  and  the  comparison  is  made  between  the  modified  light  under 
study  and  the  full,  direct  sunlight.  Since  the  luminosity  of  the 
latter  is  variable,  the  results  are  not,  of  course,  in  absolute  term-, 
even  if  the  subjective  error  were  eliminated. 

The  Sharpe-Millar  (73)  photometer  is  a  more  recent  development 
and  probably  superior  to  other  photometers  using  comparison  Lamps, 
in  that  the  light  is  supplied  by  electric  current,  and  the  comparison 
lamp  may  be  standardized  as  often  as  necessary  by  varying  the 
amperage.  Kimball  (63),  however,  in  using  it  through  a  great 
range  of  daylight  values,  found  it  necessary  to  have  screen-  to  cut 
down  the  intensity  of  the  daylight  to  the  range  of  the  artificial  light; 
also  blue-grass  screens  to  reduce  the  lamplight  to  the  color  of  day- 
light (skylight)  alone.  It  is  thus  seen  that  a  comparison  of  sun- 
light with  artificial  light  has  various  complicating  factors,  even 
with  the  best  of  photometers. 

With  certain  correction  factors  arising  from  the  use  of  these  screens 
the  distance  of  the  comparison  lamp  from  the  photometric  device 
is  made  to  express  directly  the  power  of  the  illuminating  source 
in  foot-candles. 

9.  Spectroscopic  measurements?'~-Yn  the  spectroscope  the  rays  of 
any  light  are  separated  according  to  Wave  Length.  This  naturally 
makes  it  possible  to  note  the  presence  or  absence  of  those  wave- 
lengths which  are  known  to  be  essential  to  the  plants  under  con- 
sideration, so  that  spectroscopic  observations  promise  much  to  the 
student  of  ecology.  Unfortunately,  however,  with  the  ordinary 
spectroscope,  observations  must  be  ocular  and  confined  to  the  visible 
or  middle  portion  of  the  spectrum.  Both  the  highly  active  chemical 
region  of  the  ultra-violet  and  the  strong  heating  rays  of  the  infra- 
red, are  outside  of  observation. 

Zederbauer  (79)  made  spectroscopic  observations  of  the  light  m 
the  forest,  from  which  he  concluded  that  there  is  a  marked  difference 
between  the  absorption  by  pine  and  spruce,  or  intolerant  and  toler- 
ant species,  respectively.  The  former  absorb  more  strongly  near 
the  red  end  of  the  visible  spectrum;  the  latter  more  strongly  in  the 
violet  region.  While  Zederbauer's  observations  and  his  attempt  to 
reproduce   the   transmitted   wave   lengths   separately   by   mean-    of 

« The  present  discussion  is  necessarily  very  sketchy,  because  it  is  Largely  suggestive  ol  iher 

ual  accomplishments  in  ecological  work.    For  a  complete  discussion  of  spec!  n 
sibil  must,  refer  the  reader  to  such  a  monograph  as  Baly's  " Spectroscopj 


RESEARCH  METHODS  IN  STUDY  OF   FOREST  ENVIRONMENT.         57 

monochromatic  glass  plates  did  not  lead  to  any  precise  results,  they 
are  certainly  suggestive  of  the  need  for  spectroscopic  measurements, 
if  we  are  to  determine  with  any  degree  of  accuracy  the  kind  of  light 
available  in  the  forest. 

It  is  self-evident  that  the  simplest  means  for  examining  into  the 
absorption  of  various  wave  lengths  by  leaves  is  to  examine  the  spectra 
of  beams  of  light  which  have  passed  through  individual  broad  Leaves 
or  layers  of  needle  leaves,  charting  the  marked  bands  of  absorption, 
and  comparing  such  charts  with  similar  ones  for  uninterrupted  sun- 
light. Likewise  the  spectrum  of  the  diffuse  light  in  the  forest  may  be 
examined,  while  the  spots  of  direct  light  which  have  reached  the 
ground  through  large  or  small  openings  may  be  expected  to  show 
essentially  the  same  character  as  light  in  the  open. 

Such  observations,  while  doubtless  of  great  value  in  gaining  an 
insight  into  the  difference  between  species  and  representing  the  first 
work  which  one  would  naturally  undertake  in  spectroscopy,  have 
only  limited  value  because  of  the  difficulty  in  reducing  the  absorption 
evidence  to  quantitative  terms.  There  would  naturally  be  alsc >  a  La i 
subjective  error. 

10.  Spectro  photographs. — Photographs  of  the  various  spectra 
which  may  be  examined  in  forest  studies  obviously  have  an  advantage 
over  mere  observations  in  their  permanency,  and  over  drawings  in 
their  completeness.  According  to  Baly  (52),  ordinary  photographic 
dry  plates  are  fairly  sensitive  to  rays  within  the  lengths  2,200  to  5,000 
Angstrom  units,  or  from  about  the  limit  of  the  blue  well  into  the 
ultra-violet.  In  the  "  orthochromatic "  plates  and  films  of  commerce 
the  tendency  toward  very  rapid  action  in  the  ultra-violet  region  is 
suppressed  by  the  use  of  dyes,  so  that  the  shades  and  tone-  of  the 
visible  spectrum  are  more  clearly  brought  out. 

Plates  approaching  monochromatic  value  have  been  prepared  for 
several  regions,  the  principle  being  in  all  cases  to  stain  the  plate  with 
a  dye  which  absorbs  strongly  the  rays  it  is  desired  to  bring  out.  Thus 
a  red-colored  dye  may  be  used  to  bring  out  yellow  and  green.  Ac- 
cording to  Baly  (52)  again,  Abney  succeeded  in  preparing  a  phot 
graphic  emulsion  which  was  sensitive  in  the  infra-red  to  20,000 
Angstrom  units,  and  the  solar  spectrum  was  actually  photographed 
to  10,000  Angstrom  units.  Such  plates,  of  course,  are  short  lived, 
being  very  sensitive  to  heat. 

In  addition,  there  are,  more  recently,  so-called  panchromatic  plates, 
which  have  a  very,  wide  range  of  sensitiveness. 

Until  more  is  known  as  to  the  part  which  the  infra-red  rays  play 
in  the  chemical  activities  of  the  plant,  it  would  seem  to  be  the  part 
of  wisdom,  in  spectrographic  observations,  to  use  several  plates,  cover- 
ing the  entire  range  of  the  spectrum  with  as  great  thoroughness  as 
possible. 


58  BULLETIN   1059,   U.    S.   DEPARTMENT   OF   AGRICULTURE. 

11.  Finally,  in  following  out  this  line  of  thought,  the  spectra- 
bolometer  represents  the  present  limit  of  thoroughness.  By  measur- 
ing the  heat  energy  of  every  part  of  the  spectrum,  all  of  the  results 
may  be  expressed  in  the  same  absolute  terms,  whereas  the  result- 
obtained  by  photochemical  reactions  must  be  in  different  terms  for 
each  of  the  several  reactions  which  are  required  to  cover  the  spec- 
trum. Furthermore,  bolometric  observations  permit  the  comparison 
of  deficiencies  in  any  region  with  deficiencies  in  the  whole  energy  of 
the  light  as  determined  by  the  same  means.  As  has  been  pointed 
out.  the  heat  energy  in  the  region  of  greatest  chemical  activity  is 
small;  but  it  is  not  too  small  for  precise  measurement,  and  when 
once  measured  it  may  be  transposed  to  terms  of  definite  chemical 
reactions,  the  transposition  factor  varying,  of  course,  with  each 
wave  length  and  with  each  reaction  considered. 

The  Langley  bolometer  has  been  briefly  mentioned,  because  it  was 
designed  for  measurements  of  the  whole  energy  of  sunlight.  The 
following  description  from  Baly  (52)  indicates  the  manner  in  which 
the  same  principle  was  adapted  to  the  most  minute  quantities. 

In  his  final  work  upon  the  solar  spectrum.  Langley  made  use  of  a  new  apparatus;7 
the  light  from  a  20-inch  siderostat  passed  through  the  slit  of  a  horizontal  collimator, 
which  possessed  a  lens  of  rock  salt  17  centimeters  clear  aperture  and  L0  meters  focal 
length.     This  lens  focused  the  ray  upon  a  prism  or  grating;  the  prism  was  of  rock 
salt,  and  was  18.5  centimeters  high  and  12  centimeters  deep  in  the  face,  and  Lad  a 
refracting  angle  of  60°.     The  angular  width  of  the  bolometer  thread  was  decreased 
to  2  inches  of  arc  by  using  a  telescope  lens  of  5  meters  focus;  the  sensitiveness  was 
thereby  increased,  and  by  improvements  in  the  galvanometer  the  apparatus  was 
made  capable  of  detecting  a  temperature  change  of  0.000001°  C.     The  whole  spec- 
trometer was  of  the  fixed-arm  type,  and  the  spectrum  was  made  to  pass  over  the 
bolometer  strip  by  rotating  the  prism.     An  automatic  self-registering  method  was 
adopted  of  recording  the  galvanometer  readings.     The  spot  of  light  reflected  from 
the  galvanometer  mirror  was  focused  upon  a  broad  strip  of  photographically  sensitive 
paper.     This  paper  strip  was  caused  to  move  slowly  in  a  vertical  direct  ion.  and  in 
this  way  a  faithful  record  of  the  excursions  of  the  light  spot  was  obtained.     At  the 
same  time  the  prism  was  slowly  rotated,  and  therefore  this  record  clearly  sin. wed  all 
the  temperature  changes  of  the  bolometer  as  the  spectrum  passed  over  it.     Further, 
the  motions  of  the  sensitized  paper  and  the  prism  were  exactly  coordinated,  so  that 
the  angular  position  of  the  prism  corresponding  to  any  portion  of  tin-  galvanometer 
record  could  at  once  be  obtained.     In  this  way,  since  the  dispersion  of  the  prism 
was  already  known,  the  wave  length  of  any  spectrum  line  shown  upon  the  record 
could  be  found,  and  also,  from  the  length  of  the  throw  of  the  light  spot,  its  intensity 
estimated.     The  delicacy  of  this  apparatus  was  sufficient  to  show  the  I>  linos  widely 
separated,  with  the  nickel  line  in  between.     *    *    * 

o   By  means  of  this  apparatus,  Langley  mapped  the  solar  spectrum  as  far  as  55  000 
Angstrom  units,  and  observed  700  lines  between  A  and  this  limit. 

12.  Evaporimeters  may  be  used  for  a  very  rough  measure  of  the 
heatmg  value  of  sunlight.  At  first  thought  it  would  seem  thai  the 
rate  of  evaporation  would  be  an  almost  ideal  measure,  since  the 

7  Brit.  Ass.  Rep.,  1894,  p.  465;  and  Nature,  51,  12  (1894). 


RESEARCH  METHODS  IN  STUDY  OF  FOREST  ENVIRONMENT.         59 

evaporation  of  a  gram  of  water  requires  a  nearly  constant  a. .muni 
of  heat,  varying  according  to  well-known  laws.  The  use  of  evapo- 
rimeters, however,  has  many  complicating  factors,  principal  among 
which  is  the  air  itself  as  a  source  of  heat.  If  the  atmosphere  is 
lacking  in  moisture  and  the  wind  movement  rapid,  even  an  evap 
rimeter  in  the  sun  may  be  cooler  than  the  air  and  consequently 
derive  heat  from  the  air.  On  the  other  hand,  when  the  rate  of 
evaporation  is  slow,  the  evaporimeter  may  be  superheated,  and  some 
of  the  radiant  energy  absorbed  will  be  dissipated  into  the  air  l>\ 
radiation. 

The  situation  is  by  no  means  simplified  by  the  use  of  a  pair  of 
evaporimeters,  one  of  which  is  designed  to  absorb  little  of  the  radia- 
tion and  the  other  much  or  all  of  it.  In  this  combination  one 
instrument  may  be  giving  heat  to  the  air  and  the  other  has  heal 
conducted  to  it. 

It  therefore  appears  that  evaporimeters  may  only  give  the  broad- 
est possible  comparison  of  light  intensities,  as,  for  example,  when  a 
number  of  similarly  constructed  instruments  are  exposed  to  similar 
atmospheric  conditions.  The  latter,  of  course,  are  very  likely  to  be 
modified  by  the  same  factors  that  modify  the  light.  For  these 
reasons  the  method  can  not  be  recommended  as  an  aid  in  the  study 
of  present  problems. 

Instruments  and  Approximate  Costs.8 

Angstrom  py  rholiometers .... 

Callendar  pyrheliometers 

Marvin  pliyrheliometer — not  on  market.     (U.  S.  Weather  Bureau) .... 

Smithsonian  pliyrheliometer  (mercurial  thermometer).     (Smithsonian 

Institution) U :( ■ 

Sharpe-Millar  photometer 100.00 

( Elements  photometer 

Exposure-meters.     (Photographic  supply  houses) L  to  5. 00 

Spectroscopes -'"  '"  100 

Thermo nn  trie  sunshine  recorders: 

Sunshine  recorder,  electric,  glass  (not  filled),  G.  S.  S.  Xo.  L2252.  - 

Electrical  sunshine  recorded,  complete 

Extra  glass  parts,  mounted  in  brass  socket,  ready  for  attaching  to 

support 

Registering  instruments  for  use  with  thermometric  recorders: 
Two-magnet  registers— 

Xo.  1.  For  sunshine  and  rainfall  (using  Form  No.  L015   B 
Xo.  2.  For   wind   velocity   and   sunshine   (using    Form    No. 

1015-C) - l26J 

\o  4    For  wind  velocity,  rainfall,  and  sunshine  (using  Form 

Xo.  lOlo-E) 

Quadruple  register  complete  (for  wind  direction,  wind  veloc- 
ity, rainfall,  and  sunshine),  with  a  years  supply  of  blank 
Forms  1017,  pens,  and  ink ** 


8  These  should  not  be  taken  as  quotations. 


60  BULLETIN   1059,   U.    S.   DEPARTMENT   OF   AGRICULTURE. 

PRECIPITATION. 

Precipitation  should  be  measured  at  as  many  special  stations  as 
possible,  but  only  at  those  which  are  fairly  permanent.  In  general, 
the  regular  Weather  Bureau  data  collected  at  a  large  number  of  sta- 
tions will  suffice  for  the  purposes  of  forest  investigators.  Because 
of  the  difficulty  of  obtaining  an  average  exposure  under  canopies, 
precipitation  should  always  be  measured  in  more  or  less  open  -it  na- 
tions, or  above  the  crowns,  except  of  course  when  it  i-  desired  to  de- 
termine the  amount  intercepted  by  crowns  (89). 

Since  precipitation  has  no  important  action  on  plants  until  it  i- 
added  to  the  moisture  of  the  soil,  there  can  be  no  object,  in  a  biologi- 
cal study,  and  especially  in  a  study  of  forests,  in  analyzing  precipi- 
tation data  verv  closelv.  For  this  reason  there  is  no  need  of  hourly 
precipitation  records  except  possibly  in  a  few  Localities  t<»  study  the 
general  character  of  the  storms,  which,  of  course,  will  vary  only 
slightly  with  the  forest  types.  For  this  purpose  the  tipping-buckel 
rain  gauge  (93)  should  be  used.  Standard  eight-inch  rain  gaug 
(93),  if  properly  exposed,  will  serve  in  most  cases,  though  more 
valuable  results  will  be  secured  where  it  is  possible  to  install  shielded 
gauges.  On  the  whole,  however,  the  gain  in  catch  through  the  use 
of  the  Marvin  shielded  gauge  9  is  hardly  of  enough  significance  t<> 
justify  the  additional  expense  of  the  installation,  at  leasl  Uw  any 
practical  benefit  to  ecology.  The  methods  of  measuring  precipitation 
are  too  well  known  to  need  description. 

Under  certain  circumstances,  as  in  situations  which  can  not  be 
conveniently  visited  every  day.  it  is  possible  t<>  increase  considerably 
the  value  of  the  record  by  keeping  some  kerosene  in  the  rain  gauge, 
which  will  cover  the  water  and  in  large  measure  prevent  it-  loss  by 
evaporation.  In  this  event  it  will  be  desirable  either  bo  pour  off  the 
kerosene  before  attempting  to  measure  the  water  or  to  pour  both 
into  a  glass  graduate  in  which  the  amount  of  water  can  be  seen  in 
a  few  moments,  after  which  as  much  of  the  kerosene  a-  possible  may 
be  replaced  in  the  gauge.  This  method  needs  little  modification  for 
the  winter  period,  if  the  snow  is  melted  before  measuring,  as  ordi- 
narily it  would  be.  It  is,  however,  very  desirable  to  have  the  snow, 
as  it  melts  naturally,  drop  into  a  seamless  basin  containing  some 
kerosene.  This  may  be  accomplished  by  placing  a  loose  funnel 
near  the  bottom  of  the  gauge. 

Exposure  of  Gauges. 

VUiile  the  measurement  of  precipitation  in  gauges  i-  ray  simple, 
the  securing  of  a  true  "catch"  is  much  more  dimcult,  and  for  this 
reason  the  greatest  care  should  be  used   to   install   gauges   in   such 

igned  by  the  present  Chief  of  the  United  States  Weather  Bureau. 


RESEARCH  METHODS  IN  STUDY  OF  FOREST  ENVIRONMENT.         61 

positions  that  a  near  approach  to  a  true  catch  will  be  aecured. 
Wind  (81)  is  the  factor  which  usually  prevents  all  of  the  precipita- 
tion of  a  given  area  from  entering  the  gauge  and  which  sometimi 
removes  snow  from  the  gauge  after  being  caught.  To  obtain  pro- 
tection from  wind"  without  obstructing  the  fall  of  precipitation  from 
any  angle,  should  be  the  chief  aim  in  the  installation  of  gauges. 
The  ordinary  rule  is  that  the  edges  of  the  shielding  objects  should 
be  at  an  elevation  of  30°  or  less  from  the  edges  of  the  gauge.  This 
rule  may  be  varied  somewhat.  Where  precipitation  is  usually  ac- 
companied by  high  winds,  the  angle  should  be  even  less  than  30  , 
and  the  shield,  to  compensate,  must  be  the  tighter.  Where  pre- 
cipitation is  not  so  likely  to  be  driven  by  wind,  the  angle  may  safely 
be  greater.  A  solid  shield  is  less  valuable  than  a  partial  one.  he- 
cause  it  may  set  up  eddies  in  the  air  currents  which  will  he  fully 
unfavorable  as  the  direct  wind.  On  the  whole,  shields  consisting  of 
trees  or  brush  are  best. 

Snow  Depths. 

The  depth  of  snow  and  its  water  equivalent  will  serve  a  useful 
purpose  in  giving  data  on  the  period  of  dormancy  for  each  forest 
type,  and  in  indicating  the  amount  of  precipitation  available  at  the 
beginning  of  the  growing  season,  which  in  some  localities  may  be 
the  larger  part  of  the  precipitation  for  the  whole  year  (82,  ss 
The  period  of  dormancy  might  be  obtained  more  exactly  by  tempera- 
ture measurements,  but  the  latter  are  not  possible  at  present,  at 
least  on  so  large  a  scale.  No  data  that  can  now  be  obtained  will 
cover  the  forest  types  of  the  mountain  regions  so  completely  as  the 
snow  depths  records,  and  the  conclusions  which  may  he  drawn  from 
them,  as  to  the  water  supply,  will  be  extremely  broad  and  com- 
prehensive. 

Snow  Scale  Readings. 

The  work  of  obtaining  snow  depth  and  density  measurements  by 
the  national  forest  ranger  force,  and  in  cooperation  with  the  Weather 
Bureau,  is  already  well  organized  in  some  localities,  and  this  work 
is  of  a  nature  which  may  be  done  creditably  by  the  general  forest 
forces.  The  same  organization  may  possibly  be  effected  with  profit 
in  other  localities.  While  the  work  was  originally  designed  to  fur- 
nish data  on  the  water  available  for  stream  flow  and  with  that  object 
in  view  has  been  discontinued  at  the  end  of  April  each  year,  there 
is  no  reason  why  it  should  not  be  slightly  extended  so  as  to  serve 
the  purposes  of  any  forest  investigations. 

The  general  plan  of  the  work  is : 

1.  To  have  a  large  number  of  snow  scales  distributed  over  ,11  the 
main  watersheds  and  throughout  the  entire  range  ol  elevations  and 


62  BULLETIN   1059,   U.    S.   DEPARTMENT   OE   AGKICULTURE. 

the  entire  range  of  types,  read  at  the  end  of  each  month  by  the 
rangers  on  whose  districts  they  are  located.  The  major  portion  of 
these  scales  are  in  timbered  areas  because  of  the  necessity  of  some 
protection  to  secure  a  representative  snow  cover  (90).  Many  of 
them,  however,  are  not  actually  in  the  forest  but  in  small  parks  or 
openings,  and  some  are  in  patches  of  aspen  or  coniferous  reproduc- 
tion. The  cover  conditions  are  classified  as  (a)  normal  forest  cover 
of  mature  or  nearly  mature  trees,  (b)  partial  cover,  as  given  by 
aspen,  reproduction,  or  scattered  trees  which  shade  the  snow  to  some 
extent,  and  (c)  no  cover,  as  in  parks  or  openings.  For  each  snow 
scale  a  complete  description  of  the  cover  and  surroundings  is  ob- 
tained. The  essential  features  of  this  description  are  listed  on  the 
record  card  for  each  scale,  and  the  cards  are  filed  serially  according 
to  scale  numbers,  each  card  carrying  the  depth  and  density  record 
for  12  years. 

2.  The  reports  from  rangers  are  submitted  at  the  end   of  each 
month  on  postal  cards,  of  which  the  following  is  a  sample: 

Snow  Scale  No.  — . 

Snow  Report National  Forest,  for  the  end  of 19 — ,  county 

,  main  drainage  ,  local  stream . 

Depth  at  scale, inches.     Average  in  vicinity, inches. 

Is  depth  more  (+)  or  less  (  — )  than  normal  for  this  time  of  year? 


Density  measurement:  Depth  of  snow  in  tube, (inches  and  tenths);  water 

equivalent  of  tube  contents, (inches  and  hundredths,  aa  shown  by  spring 

balance). 

^Density  estimate,  — per  cent.     (Light,  fresh  snow  should  be  estimated  at  6  to  8 

per  cent;  settled,  dry  snow  at  8  to  15  per  cent;  drifted,  compact  snow  at  15  to  20 
per  cent;  frozen  or  wet  snow,  with  ice  at  bottom,  at  20  to  40  per  cent 

The  above  observations  were  made  by ("myself  '  or  name  of  other  party  | 

on 19—. 

(Signature) 

Fun  si    J!<inr;,r. 
*  Not  to  be  filled  by  officers  having  density  apparatus. 

3.  Snow  depths  are  read  at  each  scale  by  simply  sighting  over  the 
general  surface  of  the  snow  and  noting  the  intersection  of  this  plane 
with  the  graduations  on  the  scale. 

4.  For  most  localities,  the  density  of  the  snow  is  estimated  from 
the  descriptive  data  given  on  the  card.  Each  ranger  is  expected  to 
make  occasional  rough  tests  to  determine  density,  so  that  lie  will 
become  proficient  in  estimating  under  varying  conditions.  A  few 
rangers  are  now  furnished  with  density-measuring  apparatus  (91) 
and  the  number  of  such  apparatus  is  to  be  increased  as  conditions 
and  funds  warrant.  The  apparatus  consists  simply  of  a  tube  in 
which  a  core  of  snow  may  be  taken  (its  length  being  noted  on  the 
scale  outside  the  tube),  and  a  balance  graduated  for  inches  of  water. 
The  weight  in  inches  divided  by  the  depth  in  inches  gives  the  densil  y. 


RESEARCH  METHODS  IX  STUDY  OE  FOREST  ENVIRONMENT. 

Where  density  *is  estimated,  the  depth  and  density  are  entered 
when  the  report  is  received,  and  the  water  equivalent  may  be  com- 
puted at  any  time.  Where  density  apparatus  is  used,  it  is  necessary 
to  compute  the  density  and  apply  this  figure  to  the  depth  reading  at 
the  scale,  which  may  be  somewhat  different  from  the  depth  noted 
on  the  tube. 

It  is  self-evident  that,  while  the  progress  of  snow  accumulation 
throughout  the  winter  is  interesting,  the  most  important  data  are 
those  which  show  the  maximum  accumulation  just  prior  t<»  the 
beginning  of  rapid  spring  melting. 

Tabulation. 

The  form  for  "Daily  and  Hourly  Precipitation'    may  serve  ;i-  n, 
monthly  summary  both  for  daily  observations  and  for  notations  from 
hourly  records  where  these  are  obtained.     The  following  should  b 
tabulated  from  daily  observation: 

Total  precipitation  in  inches  of  rain  or  melted  snow. 

Unmelted  snow,  as  measured  in  the  gauge,  in  snow  bin-  8  <>r 
on  the  ground.  Precise  measurements  of  the  snowfall  appeal-  to  !».■ 
useless  to  the  ecologist. 

Depth  of  snow  on  the  ground  at  time  of  observation. 

Number  of  storms  of  rain,  sleet,  or  snow  which,  by  the  weight  of 
accumulated  water  or  because  of  accompanying  wind,  do  mechanical 

damage  to  trees. 

The  following  data  are  merely  intended  to  depict  the  character 
of  storms,  and  should  be  obtained  from  the  hourly  automatic  records 
for  a  few  stations  typical  of  the  different  regions  and  altitudes: 

Number  of  hours  having  measurable  precipitation. 
Number  of  hours  having  0.05  to  0.10  inches  precipitation. 
Number  of  hours  having  0.10  to  0.20  inches  precipitation. 
Number  of  hours  having  0.20  to  0.50  inches  precipitation. 
Xumber  of  hours  having  more  than  0.50  inches  precipitation. 

I       The  "Summary"  form  will  serve  as  an  annual  summary  for  pre- 
cipitation data  of  both  classes  and  will  include  all  of  the  .lata  given 
as  the  monthly  sums  or  means  on  the  several  "Daily  and  I  [ourlj  I  re 
cipitation"  forms.     As  usual,  sums  and  means  should  1-  computed 
for  the  growing  season  as  well  as  for  the  whole  year. 

Instruments  and  Approximate  PRiens. 

Rain  and  snow  gauges:  _  nn 

Rain  and  snow  gauge,  8-inch  Weather  Bureau  pattern  5.  30 

Supports,  box,  for  rain  and  snow  gauge.       .  -  ^  _ 

Measuring  sticks,  rain  gauge,  cedar,  No.  1--' 
Rain  gauge,  tipping-bucket,  with  supports  and  measur-  ^ 

ing  tube " 


64 


BULLETIN   1059,   TJ.    S.   DEPARTMENT   OE    AGBI<?UL,TUHE. 


Ci 


o 


8 


a 

© 


O 

— 


o 

w 

- 

o 
M 

Q 

!h 

i— i 
< 


o 


.o 


o 
o 


o 
o 


so 


S5, 


OS 

E 

u 
O 


No.  wind, 
rain,4  sleet  or 
snow  storms 

doing 

mechanical 

damage 

to  trees. 

From  hourly  automatic  records. 

Total  hours  with— 

More  than 
0.50"  per  hr. 

0.20-0.50" 
per  hr. 

0.10-0.20" 
per  hr. 

0.05  0.10" 
per  hr. 

Precipitation. 

Daily  observation.1 

Snow'1  on 

ground  at 

obs.  (inches). 

~. 

— 
2 

Snow  un- 

incltcd" 

(inches). 

Total  precipi- 
tation (inches). 

— 

- 
— 

Q 

~ 

K 

— 

■- 

EC 

1- 

/ 

a 

a 

— 
C 



- 

-- 

— 

IT. 

- 

t- 

X 

3 

= 

- 
— 

r 

— 
z 
— 

- 
- 

~ 

— 

- 

RESEARCH  METHODS  IN  STUDY  OF  EOREST  ENVIRONMENT.  65 


' 

S 

Js) 

■ 

: 

• 

: 
• 

r 

c 

d 

03 
« 

E 

s- 
O 

"5 
+j 
o 
+■> 

:    8 

.    P 

> 

-* 
< 

• 

a 
4 

03 
1* 


03 

© 

-*j 

TJ 

H 

© 

O 

© 
co 

O 


c3 
© 

o 


CO 

co 


© 
,4 

>> 

© 
u 

CO 


© 

ca 
© 

a 
© 

>> 

S3 

a 

© 

03 


E     • 

a 

fl'S 

<A 

»3 
■G  5 

-o 

o 

+J   t£ 

+J 

'S  ° 

a 
© 

s*- 

H* 

ftfl 

© 

ro° 

© 

T3T3 

A 

i_  © 

O  co 

©  3 

uT 

©  ^ 

d 

t>  © 

c? 

a  a 

© 
o 

v. 

e 
a, 


5     ft 


£--  $  c 
o  g  o2 

.2=3  6  ^s 

to  (-.  ©  03  t_ 

03   u,jC    r-    C 

>"  ©  +j  c    - 

>  +J    © 

6X    .  co 


mo 


2  CO 


CO  - 

©  C© 

a—  -  33  - 


© 
b  a 

0.5 


**>  c/T 


CO    --I 

A  ©  o 

a  3^ 

•  fH    iH    N'T    Li 

g-gojjo 

o°  3  p© 

O  03  r-.S^D 

wo5o3  g 

o 


■-I   «<  ro   ->■ 


66  BULLETIN   1059,   U.   S.   DEPARTMENT   OF  AGRICULTURE. 

SOIL   MOISTURE    AND    SOIL    QUALITIES. 

The  subject  of  soil  moisture  is  closely  related  to  that  of  precipita- 
tion. Since  the  physical  and  chemical  properties  of  the  soil  are 
closely  linked  up  with  moisture,  it  seems  logical  to  consider  all  of 
these  subjects  together  as  a  question  of  water  supply,  after  which 
atmospheric  conditions  which  particularly  affect  water  losses  may 

be  taken  up. 

There  is  practically  no  question  that  water  is  the  prime  requisite 
of  all  life,  for  without  waiter  the  colloids  could  not  exist.  It  is 
hardly  more  true  of  plants  than  of  animals  that,  besides  possessing 
water  at  any  given  time,  they  must  be  almost  continually  given  new- 
supplies  to  make  up  for  unavoidable  losses;  but,  with  the  exception 
of  aquatic  species,  plants  are  more  at  the  mercy  of  the  moisture  of 
the  habitat  than  are  animals,  because  they  can  not  move  to  new 
supplies — -the  water  must  somehow  be  brought  within  their  reach. 

Ecologically,  it  is  perhaps  unsafe  to  say  thai  moisture  has  more 
to  do  with  the  establishment,  development,  and  succession  of  a  plant 
society  than  any  other  condition,  that  is,  that  it  controls  the  character 
of  the  plant  society  more  directly.  It  is  perhaps  nearer  the  truth  to 
say  that  when  the  temperature  conditions  are  about  optimum  for  a 
given  plant  or  society,  moisture  determines  success  or  failure  almost 
absolutely.  Yet  this  does  not  express  the  situation,  for  in  a  vast 
majority  of  cases  the  plant  society  must  depend  at  all  stages,  hut 
particularly  at  its  initiation,  upon  a  proper  balance  between  tempera- 
ture and  moisture,  especially  as  these  are  integrated  in  the  condition 
of  the  surface  soil. 

In  the  last  analysis  all  other  environmental  condition-  react  more 
or  less  on  the  soil  moisture,  and  the  best  measure  of  their  inlluence 
in  this  respect  is  found  in  a  measure  of  changes  in  the  soil  moisture. 
It  is  readily  seen,  therefore,  that  the  direct  measurement  of  the  soil 
moisture  is  of  the  utmost  importance. 

The  moisture  content  of  the  soil,  whether  expressed  m  grains  per 
kilogram  of  soil  or  cubic  centimeters  per  cubic  meter  of  soil,  does  not 
give  directly  a  measure  of  the  rate  at  which  it  may  be  obtained  by 
the  plant,  because  of  the  great  variation  in  the  moisture-withholding 
powers  of  soils.  This  rate  is  obviously  very  important  whenever  the 
atmospheric  conditions  are  such  as  to  cause  heavy  loss  from  the 
leaves,  and  may  often  determine  success  or  failure  «.f  the  individual 
plant  and  of  the  society. 

OSMOSIS    AS    A    FACTOR   IX    WATER    ABSORPTl<  »\ . 

The  rate  of  absorption  is  unquestionably  dependent  upon  the 
simple  physical  process  known  as  diffusion,  which  is  commonly  called 
osmosis  when  speaking  of  plants,  since  the  mixing  of  the  two  Liquids 


RESEABCH  METHODS  IN  STUDY  OF  FOREST  ENVIRONMENT.         67 

is  somewhat  modified  by  a  semipermeable  membrane  between  them 
which,  in  fact,  tends  to  make  the  diffusion  one  sided.  Expressed 
briefly  and  without  attempting  to  define  the  kinetic  forces,  if  there 
is  water  on  one  side  of  the  semipermeable  membrane  and  on  the  other 
side  a  solute  in  water,  there  is  a  constant  tendency  of  the  moleculi 
of  water  to  move  both  ways  through  the  membrane,  while  the  mole- 
cules of  the  solute,  being  heavier  and  larger,  do  not  so  readily  diffuse 
to  the  side,  where  they  are  deficient  in  numbers.  On  the  other  hand 
the  presence  of  the  solute  on  one  side  of  the  membrane  evidently  has 
the  effect  of  suppressing  the  energy  of  the  water  molecules  there,  so 
that  fewer  molecules  pass  in  the  outward  direction  than  are  coming 
in  through  the  membrane.  This  results  in  an  accumulation  on  one 
side  of  the  membrane,  and,  so  far  as  the  increasing  volume  here  is 
restrained,  gives  rise  to  a  pressure  which  is  termed  ''osmotic  pres- 
sure." Actually,  the  pressure  must  be  due  to  the  energy  of  the  free 
molecules  which  bombard  the  membrane  from  the  outside. 

In  the  case  of  the  plant,  there  are,  in  addition  to  the  mineral  salts 
and  organic  compounds  which  may  be  in  solution  within  the  root- 
cells,  and  within  each  succeeding  cell  in  greater  concentration  from 
root  to  leaf  tip,  two  additional  factors  or  forces,  which  undoubtedly 
have  much  to  do  with  osmosis.  While  the  forces  may  be  called  capil- 
larity and  adsorption,  respectively,  there  is  no  reason  for  supposing 
that  the  action  of  these  forces,  so  far  as  water  molecules  are  con- 
cerned, is  different  from  that  of  molecules  of  solids  in  solution.  In 
other  words,  a  molecule  of  a  solid,  whether  acting  individually  as  part 
of  a  relatively  solid  mass  or  surface  or  as  part  of  a  gelatinous  ma- 
like  the  cell  protoplasm,  by  its  gravitational  attraction  for  a  water 
molecule  tends  to  suppress  the  activity  of  that  molecule  and  thereby, 
under  the  proper  conditions,  gives  rise  to  the  process  we  call  osmosis. 

The  so-called  capillary  attraction  of  the  cell  walls,  which  cause 
them  to  imbibe  enough  water  to  fill  their  intercellular  spaces.  can 
not  be  considered  an  important  factor  in  osmosis,  for  ordinarily  this 
attraction  would  pull  the  water  as  strongly  from  one  side  of  the 
wall  as  the  other.  In  the  case  of  dead  wood  cells,  it  may  be  imagined 
that  a  small  amount  of  water  is  transferred  from  one  point  to  another 
through  no  other  action  than  this  capillary  affinity  of  the  cell  walls. 

The  really  important  element  in  osmosis  is,  without  question,  the 
affinity  of  the  protoplasm  for  water.  While  it  appears  to  be  true  that 
a  cell  similar  to  a  plant  cell  might  have  the  same  water  content,  aris- 
ing from  osmotic  pressure  and  due  entirely  to  inorganic  solutes,  still 
the  important  thing  is,  not  merely  that  the  protoplasm  must  have  a 
certain  minimum  amount  of  water  to  maintain  its  properties  as  a 
colloid,  but  that  through  selective  absorption  it  is  able  to  regulate,  at 
least  to  some  extent,  the  nature  of  the  solution  which  shall  occupy  I  he 
space  within  the  cell.     The  protoplasm,  while  directly  effective  in 


68  BULLETIN   1059,   U.    S.   DEPARTMENT   OF   AGRICULTURE. 

causing  osmosis,  is  undoubtedly  regulatory  of  all  the  conditions 
within  the  cell.  It  is,  indeed,  through  the  study  of  soils  that  the 
present  conception  of  the  nature  of  osmosis  and  of  the  importance  of 
'the  colloids  of  the  plant  in  this  process  is  arrived  at.  The  work  of 
Bouyoucos  (109)  followed  by  Hoagland  (127)  has  been  especially 
instructive  on  this  point.  Bouyoucos  and  McCool  (106)  first  at- 
tempted to  determine  directly  the  concentration  of  soluble  salts  in 
the  soil  by  measuring  the  freezing-point  depressions  of  soils  in  their 
natural  conditions  of  moisture  and  with  definite  amounts  of  water 
'added  to  them.  At  the  outset,  no  doubt,  he  supposed  that,  allowing 
for  increased  solubility  of  the  soil  substances  with  increased  moisture, 
the  freezing-point  depressions  would  increase  proportionately  as  the 
concentration  of  the  solutes  increased  by  reduction  of  the  whole 
moisture  of  the  soil.  This  he  found  not  to  be  the  case,  for,  while  the 
effect  of  increasing  concentration  was  shown  in  greater  freezing-point 
depressions  as  the  moisture  was  reduced,  a  point  was  rather  suddenly 
reached  at  which  no  freezing  at  all  occurred.  From  this  Bouyoucos 
concluded  that  there  must  be  at  all  times  in  the  soil  a  certain  amount 
of  u  unfree"  water,  probably  not  in  a  liquid  state  but  so  adsorbed  or 
chemically  combined  that  at  no  time  does  it  constitute  a  part  of  the 
soil  solution.  This  water  was  conceived  to  be  that  which  is  held 
within  the  colloidal  or  clay  masses  of  the  soil,  but  it  is  fairly  evident 
that,  since  pure  sand  may  contain  a  small  amount  of  such  water,  it 
may  be  in  part  water  in  extremely  thin  films,  or  more  probably  in 
the  form  of  independent  molecules,  on  the  surfaces  of  the  crystalline 
particles. 

In  his  later  work  Bouyoucos  (109)  succeeded  in  measuring  the 
volume  of  this  unfree  or  combined  water  indirectlv,  bv  noting  the 
volume  expansion  of  the  whole  mass  at  the  instant  when  freezing 
occurred.  By  this  means  he  was  able  to  determine  just  what  propor- 
tion of  the  whole  amount  of  water  entered  into  the  freezing  process. 
Adding  in  every  case  5  cubic  centimeters  of  water  to  25  grams  of  air- 
dried  soil,  he  found  that  the  proportion  of  this  5  cubic  centimeters 
which  did  not  freeze  was  only  2  per  cent  (0.10  cubic  centimeter)  in 
the  case  of  a  quartz  sand,  but  60  per  cent  (3  cubic  centimeters)  in  the 
case  of  a  clay,  and  74  per  cent  for  heavy  silt  loam. 

The  exact  amount  of  unfree  water  for  a  given  soil  was  found  to  be 
dependent  on  several  experimental  conditions,  which  led  to  the  belief 
that  under  certain  conditions  it  could  be  transformed  into  free  water. 
In  other  words,  there  is  no  fixed  point  at  which  the  water  begins  t<» 
be  unfree,  but  under  given  conditions  each  soil  exhibits  certain  <  !<•!  in  it  e 
characteristics.  Thus  there  was  in  all  soils,  except  the  quart/  sand, 
less  water  which  failed  to  freeze  when  10  cubic  centimeters  had  been 
added  to  a  standard  sample  than  when  only  5  cubic  centimeters  were 
used.    There  was  also  less  when  the  supercooling  was  carried  below  ( he 


RESEARCH  METHODS  IX  STUDY  OF  FOREST   ENVIRONMENT.         69 

standard  temperature  of  —4°  C.,  and  when  the  process  was  repeated 
on  the  same  sample  several  times.     Without  going  further  into  th< 
details,  which  are  readily  available  in  the  original  article,  one  more 

fact  should  be  mentioned,  namely,  that  the  moisture  contents  at  which 
various  soils  fail  to  show  definite  freezing  points,  and  similarl}  the 
contents  which  by  the  method  just  described  are  found  definitelj  Dot 
to  freeze,  bear  a  close  relation  to  the  wilting  coefficients  of  the  same 
soils. 

Directly  bearing  on  the  point  as  to  the  part  played  by  the  colloidal 
masses  of  the  soil,  Bates  (105)  found,  for  eight  samples  of  Michigan 
and  Nebraska  sand,  each  taken  at  a  depth  of  1  foot,  and  consequently 
showing  a  maximum  of  2  per  cent  humus,  that  there  was  a  Linear  re- 


DIAGRAM    1 

RELATION  OF  WILTING  COEFFICIENTS 

TO 

SILT  AND   CLAY  CONTENTS 

OF 

MICHIGAN  SANDS 

3             ol-foot  samples  containing  little  humus, 
x  Surface  samples  with  more  or  less  humus 

X 

i_l 

2:  +- 

LJ  IT} 

r.  £ 

-  u  * 

— '  DO 

-_)C 
<  CD 

X 

X 

o. 

'"-if 

x^_. 

X 

-Prot 

iatote 

>-  unf 

ree  v 

rater 

*-of- 

rtean 

"sand 

0.4 

2% 

i 

2 

3 

S 

LT 

A 

MMD 

CLA 

5 

/   CC 

INTE 

NT, 

Perc 

enta 

*    t 

• 

9  . 

lation  between  the  final  wilting  coefficients  for  jack  and  Nor*  ay  pines 
on  the  one  hand,  and  the  combined  silt  and  clay  contents  of  the  i 
spective  soils  on  the  other.     The  latter  varied  from  0.3  to  6.4   per 
cent.    The  behavior  of  surface  soils  with  larger  humus  contents  s  as 
different;  but  these,  when  decidedly  lacking  in  humus,  might   gi 
even  lower  values  than  the  deep  soils,  probably  «...  account  ol  more 
thorough  leaching.    The  results,  as  shown  in  diagram  1 .  indicate 
if  silt  and  clay  were  entirely  eliminated,  the  sands  might  still  poss< 
a  wilting  coefficient  of  about  0.43  per  cent.    This  is  extremely 
to  the  value  for  unfree  water  which  Bouyoucos  found  to  be  almos 
constant  in  the  case  of  quartz  sand,  regardless  of  the  experimental 

conditions.  ,      _     ,  ,       ,     i 

The  conclusion  is  therefore  reached  that,  in  the  final  struggle  which 
determines  whether  the  plant  shall  obtain  sufficient  water  for 


70 


BULLETIN   1059,    U.    S.   DEPARTMENT   OF   AGRICULTUB 


barest  existence,  there  is  a  contest  for  water  between  two  colloidal 
masses,  either  of  which  may  be  able  to  absorb  the  solutes  which  have 
been  in  the  water  about  them,  and  hence  eliminate  osmotic  action  in 
the  ordinary  sense  of  an  interchange  between  two  liquids.  Appar- 
ently this  contest  between  the  attraction  of  the  soil  particles  and  clay 
masses  on  the  one  hand,  and  the  cell  walls  and  protoplasmic  masses 
on  the  other,  is  not  essentially  different  in  principle  from  osmosis, 
except  that  in  the  final  stage  of  the  struggle  the  movement  of  a 
molecule  of  water  from  one  side  of  the  line  to  the  other  becomes  im- 
possible because  of  the  lack  of  a  liquid  conductor.    It  is  probably  on 


DIAGRAM     2 
ILLUSTRATING    THE   COMPLEXITY 

OF    THE 

WATER- WITH  HOLDING   FORCES  IN  SOILS 

AS  SHOWN   BY    THE 

FREEZING  BEHAVIOR   OF   THE  WATER 

V) 

•o 

— tH 

o 

55 
12  Y\ 

Id 

1.0   D 

h 

z 

0.8  O 

0. 

o 
SL£Z 

si 
u 

■S^i 

la. 
0.Z 

h. 

0.0      o 

' 

=4i£o 

0 

z 

0 

w 
; 

\TER 

C0NT 
i 

:nt  o 

-   SOI 

,,per 
0  r 

cent 
.  S 

0 

i 

0 

i. 

1 

0 

this  account  that,  in  a  strong  clay  soil,  the  plant  may  wilt  consider- 
ably before  an  equilibrium  of  tensions  has  actually  been  produced. 

The  important  point,  however,  is  that  under  normal  growing  con- 
ditions in  the  plant  and  soil  there  is  a  set  of  forces  at  work  regulating 
the  supply  of  water  to  the  plant,  which  is  dependent  almost  wholly 
on  the  presence  of  solutes  in  free  water,  or  osmosis  in  the  ordinary 
sense;  while,  when  the  water  becomes  relatively  scarce  (this  may 
be  at  20  or  30  per  cent  moisture  content  in  a  clay  soil),  an  almost 
entirely  different  set  of  forces  is  brought  into  play.  It  therefore 
appears  that  the  study  of  soil  moisture  is  not  so  elemental  as  it  has 
been  supposed,  and  that  the  value  of  soil  moisture  to  the  plant  can 
not  be  expressed  by  a  direct  linear  function  of  the  amount  of  water 
in  the  soil.  Diagram  2  is  inserted  to  show  the  nature  of  the  problem. 
Forest  investigators  must  get  away  from  this  elemental  idea,  taking 
up  the  study  of  soil  moisture  at  the  point  to  which  expert  soil  physi- 
cists have  already  brought  it. 


RESEARCH  METHODS  IN  STUDY  OF  FOREST  ENVIRON  M  BNT.         7  1 
Problems  and  Some  Definition 

Such  being  the  general  situation,  it  is  evident  that  the  ecologisl 
has  a  number  of  related  problems  to  solve  before  the  measurement 
of  moisture  has  much  meaning.  In  the  following  paragraphs  cer- 
tain terms  have  been  introduced  which  will  nave  a  rather  definite 
usage  in  the  later  discussion. 

1.  The  total  moisture  must  be  obtained  as  the  basis  for  all  ex- 
pressions of  current  conditions  in  the  soil,  unless  they  are  measured 
directly  in  terms  which  will  give  osmotic  pressure 

2.  The  nonavailable  moisture  must  be  measured  with  reference  to 
the  plant  or  plants  concerned,  either  directly  by  wilting  i  or 
through  some  established  relationship  between  the  uniting  m  ">■<  lent 
on  the  one  hand,  and  the  antiosmotic  pressure  (P')  the  capillary  mo 
ture,  the  moisture  equivalent,  or  the  hygroscopic  coefficient,  on  the 
other. 

3.  The  available  moisture  may  be  expressed  as  the  difference  be- 
tween the  total  and  the  nonavailable  moisture.  Such  an  expression 
may  have  some  direct  ecological  significance  in  indicating  the  prob- 
able duration  of  the  moisture  supply  and  the  life  tenure  of  the  plan 

4.  The  availability  of  the  moisture  is  seen  in  the  general  relation- 
ship between  the  available  moisture  and  the  total  moisture,  and  may 
be  expressed  by  a  ratio  such  as  3:4,  or  by  a  decimal  such  as  0.75,  on 
the  scale  of  an  unattainable  unity. 

5.  The  coefficient  of  availability  is  a  more  exact  expression  of  tin- 
relation  between  the  osmotic  pressure  of  the  plant  (P)  and  the  anti- 
osmotic  pressure  of  the  soil  water  (P'),  and  is  a  measure  of  the  possi- 
able  rate  of  intake.  Thus,  if  the  soil  has  a  freezing-point  depression 
of  0.5°,  and  the  plant  of  1.5°,  the  respective  osmotic  pressures  are 
P'  =  6.025  and  P  =  18.04  atmospheres,  and  the  possible  rate  of  intake 
is  indicated  by  the  difference,  which  is  approximately  12.02  atmos- 
pheres. It  is  perfectly  evident,  however,  that  the  osmotic  pressure 
in  the  root  tips  may  be  very  little  greater  than  the  osmotic  pressure 
of  the  soil,  while  there  may  be  a  very  great  increase  in  passing  from 
the  roots  to  the  leaf  tips  where  water  is  being  lost  most  rapidly.  A> 
this  is  also  the  most  convenient  point  for  measuring  the  osmotic  pr< 
sure  in  the  plant,  and  such  can  here  be  accomplished  withoul  seri- 
ously disturbing  the  plant,  it  is  suggested  that  the  osmotic  pre* 

at  the  leaf  tips  should  be  the  basis  for  expressing  the  plant  condition 
In  this  event,  the  actual  availability  of  the  water  is  obviously  affected 
by  distance,  or  the  mean  osmotic  gradient  from  the  soil  to  the  leaf 
tips.  The  coefficient  of  availability  (AA)  must  therefore  be  ex- 
pressed by  ^J^-',  in  which  L  is  the  distance  in  centimeters  from  the 
root  tip,  or  point  of  measuring  the  soil  condition,  to  the  leaf  tip. 


72  BULLETIN"   1059,   U.    S.   DEPARTMENT   OF   AGRICULTURE. 

In  addition,  for  the  greater  refinement  of  this  expression,  it  will 
be  necessary  to  make  a  correction  for  the  height  to  which  the  wain 
must  be  lifted.  In  small  plants  this  would  be  of  no  consequence,  but 
with  tall  trees  it  is  evidently  the  factor  which  brings  the  coefficient 
of  availability  to  zero  long  before  the  soil  moisture  is  exhausted. 
This  correction  may  be  taken  as  approximately  0.097  atmosphere 
per    meter    of    height.     Expressing    this    whole    correction    by    G, 

It  should  be  noted  that  the  availability  of  the  moisture  is  an  ex- 
pression whose  value  will  change  only  very  gradually  with  the  ex- 
haustion of  the  soil  moisture,  and  is  really  based  upon  an  assumed 
equilibrium  between  the  osmotic  pressure  of  the  whole  plant  and  that 
of  the  soil  at  the  time  of  wilting,  which  it  is  possible  to  attain  ap- 
proximately if  the  wilting  of  the  plant  is  brought  about  very  slowly. 

On  the  other  hand,  the  coefficient  of  availability  must  be  con- 
stantly fluctuating,  being  dependent  both  on  supply  and  demand 
(loss).  Thus  the  rapid  loss  of  a  considerable  amount  of  water  from 
the  leaves  must  be  almost  immediately  reflected  in  the  osmotic  pres- 
sure there,  the  gradient  from  leaves  to  roots,  and  the  rate  of  intake 
at  the  roots  and  of  transfer  from  cell  to  cell.  The  objects  in  the  use 
of  such  an  expression  must  be  to  show  not  only  how  the  demands  of 
the  plant  vary  from  time  to  time,  but  how  nearly  the  demands  created 
by  certain  atmospheric  conditions  may  be  satisfied.  Thus  some  in- 
sight is  obtained  into  the  conditions  governing  growth-rate. 

6.  There  are  various  other  conditions  of  tin-  -oil  which  have  an 
effect  upon  plants  and  may  or  may  not  be  fully  indicated  by  the 
osmotic  pressure  of  the  soil  solution  at  any  time.  Of  these  may  be 
mentioned : 

(a)  The  hydrogen-ion  concentration  as  an  expression  of  the  de- 
gree of  alkalinity  or  acidity. 

(b)  The  make-up  of  the  soil,  and  particularly  it-  clay  emit  cut  as 
indicated  by  the  mechanical  analyses. 

(c)  Humus  content. 

(d)  The  capillary  transporting  power  of  the  soil,  by  which  water 
from  distant  regions  may  be  brought  to  the  roots.  Obviously  this 
may  often  be  an  extremely  important  factor  in  the  economy  of  the 
plant.  Its  importance  is  somewhat  minimized  when  moisture  deter- 
minations are  certainly  made  in  the  soil  area  which  is  reached  by  the 
roots. 

0)  Chemical  content  of  all  elements  and  compounds,  with  par- 
ticular reference  to  those  which  are  necessary  in  nutrition. 


RESEARCH  METHODS  IN  STUDY  OF  FOREST  ENVIRONMENT. 
Total  Moisture  Determinations. 

The  determination  of  the  current-moisture  contenl  of  the  soil  at 
a  given  point  is  an  exceedingly  simple  matter,  and  a  vast  amounl 
such  work  has  been  done  in  connection  with  agricultural  investiffa- 
tions  and  greenhouse  experiments;  in  fact,  so  much  has  been  done 
that  citations  are  useless. 

On  the  other  hand,  repeated  determinations  at   a  given  point   to 
show  changes,  minima,  etc,  immediately  introduce  complications. 
When  a  sample  has  been  taken  from  the  ground,  it  is  wry  difficult  to 
fill  the  space  with  the  same  kind  of  soil  as  before,  and  even  if  tin- 
were  accomplished  the  new  soil  would  not  soon  be  in  a  normal  mois- 
ture condition.     The  next  sample  must,  therefore,  alm<»-t  certainly 
he  taken  a  short  distance  away,  and  almost  invariably  this  int  rodua 
a  change  in  composition,  such  that  equal  moisture  contents  in  two 
successive  samples  may  not  have  the  same  plant  value.     Usually  m 
agricultural  soils  or  well-mixed  potting  soils,  these  variation-  may 
be  ignored.     Very  often  in  forest   soils,  however,   the  changes   in 
composition  are  very  abrupt;  in  fact  there  is  often  no  such  thing 
as  uniformity  of'soil  texture,  even  in  a  practical  sense.     The  sampling 
of  forest  soils,  moreover,- is  often  difficult  owing  to  the  presence  of 
rocks  which  make  it  impossible  to  obtain  a  sample  at  the  desired 
spot,  at  least  with  borers  of  any  description.     These  mechanical  diffi- 
culties may  usually  be  overcome  by  the  use  of  pick  and  shovel,  and  in 
careful  surveys  of  the  root  zones  of  individual  trees  or  group-  such 
methods  will  undoubtedly  have  to  be  resorted  to. 

In  practice,  it  is  usually  impossible  to  examine  a  large  number  of 
soil  points  with  sufficient  frequency  to  show  even  approximately 
the  changes  in  soil  moisture.  It  is  necessary  to  select,  more  or  1< 
arbitrarily,  points  which  seem  to  represent  the  average  of  conditions 
in  the  plant  formation  or  forest  type  under  study,  and  to  confine  the 
effort  to  showing  as  accurately  as  possible  all  the  condition-  which 
occur  at  this  point. 

SOIL-WELLS    FOR   REPRESENTATIVE    POINT  - 

In  view  of  what  has  been  said,  it  appears  necessary  to  make  provi- 
sion for  establishing  some  standard  conditions  under  which  soil  sam- 
ples shall  be  taken  at  permanent  stations.     The  ideal  method  would 
undoubtedly  be  to  show  the  moisture  content  of  a  smgle  sample  o 
soil  from  time  to  time,  and  it  has  been  suggested  that   this  mu 
be  accomplished  bv  the  periodic  weighing  of  a  standard  soil  sample 
contained  in  a  porous  cup  which  would  be  permanently    ocated 
the  soil  point.     This  plan  involves  a  number  of  technical  difficulties 
and  is,  moreover,  wholly  untried.     The  nearest  practical  approach 
to  the  method  of  a  single  sample  would  seem  to  be  in  the  plat,  of 


74  BULLETIN   1059,   U.    S.   DEPARTMENT   OF   AGRICULTURE. 

soil  wells,  which  has  been  thoroughly  tried  at  the  Fremont  Experi- 
ment Station  and  elsewhere. 

At  each  station  where  soil  moisture  is  to  be  determined  periodi- 
cally, a  well  18  to  24  inches  in  diameter  and  4  or  more  feet  deep  may 
be  dug.  At  the  nearest  available  point  a  soil  quarry  is  established 
for  each  station  or  group  of  stations  having  similar  soils.  From  this 
quarry  is  taken  soil  containing  only  a  moderate  amount  of  humus, 
and  which  should  be  sifted  through  4-mesh  screen.  At  the  outset 
sufficient  sifted  soil  is  obtained  to  fill  the  well  and  to  furnish  1  or  2 
cubic  feet  of  reserve,  the  whole  being  thoroughly  mixed.  The  sifted 
soil  should  be  firmly  tamped  into  the  well.  It  will  be  better  if  the 
well  may  be  allowed  to  stand  a  year  before  being  used,  the  soil  be- 
coming settled  by  water  action  and  being  to  some  extent  penetrated 
by  roots. 

In  order  to  maintain  uniform  conditions  at  the  surface,  each  soil 
well  should  be  kept  free  of  litter  by  means  of  a  frame  of  1  by  4  inch 
boards,  18  or  24  inches  square,  which  may  be  slightly  sunk  in  the 
soil.  Over  this  is  placed  a  slightly  larger  frame  covered  with  hard- 
ware cloth.  It  is  evident  that  this  frame  will  interfere  with  sur- 
face erosion.  The  surface  of  the  soil  in  the  well  should  at  all  times 
be  kept  flush  with  the  surface  of  the  ground  around,  so  that  the 
amount  of  water  available  for  absorption  is  not  appreciably  greater 
or  smaller  than  elsewhere. 

At  the  time  of  digging  any  soil  well,  samples  of  the  native  soil  at 
1,  2,  and  3  feet  from  the  surface  and  other  depths  at  which  mois- 
ture is  to  be  determined,  as  well  as  one  of  the  prepared  soil  for  the 
well,  should  be  obtained  for  testing.  Each  sample  should  comprise 
about  30  pounds  and  should  be  air-dried,  unless  it  is  to  be  used 
immediately. 

Each  soil  quarry  should  be  permanently  designated  at  the  time  of 
its  first  use,  and  a  record  may  be  made  of  the  quality  and  location  of 
material  taken  therefrom,  so  that  in  the  future  fresh  supplies  for  the 
well  may  be  obtained,  with  a  little  trial,  very  nearly  like  the  original. 

As^such  soil  wells  are  used  year  after  year,  it  will  be  noted  that 
the  finer  material  is  to  some  extent  concentrated  in  the  lower  lavers, 
especially  if  the  soil  of  the  well  is  decidedly  sandy  and  loose.  This 
will  not  be  found  so  important  a  change  if  the  soil  is  compact,  or 
contains  considerable  humus. 

The  question  naturally  arises,  how  will  the  moisture  in  one  of 
these  wells  compare  with  the  moisture  of  the  native  and  undisturbed 
soil  on  either  side?  For  the  reason  that  the  soil  of  the  well  is  quite 
certain  to  be  finer  than  the  native  forest  soil,  it  is  evident  that  the  well 
soiljwill  always  contain  a  higher  percentage  of  moist  inc.  Further- 
more, without  going  into  all  the  details,  this  question  is  answered 
unequivocally  by  saying  that  the  moisture  content,   as  determined 


RESEARCH  METHODS  IN  STUDY  OF  FOREST  EtfVIKONMENT.         75 

for  the  well  soil,  can  never  be  considered  as  an  exacl  measure  of  the 
moisture  outside.  The  well  samples  will  be  principally  useful  to 
showing  changes,  and  without  doubt  should  occasionally  be  compared 
with  native  samples  taken  near  by.  It  is  believed,  however  tin,  for 
practical  purposes  a  certain  constant  relation  between  ,1.,  two  soils 
may  be  assumed.  So  far,  because  of  the  great  difficulty  of  actual 
contact  tests  between  two  soils,  the  moisture  ratio  al  equilibrium 
must  be  established  on  theoretical  consideration^ 

From  what  is  known  of  capillary  movement  in  soil.  (116)  ,,  w,lU|,l 
seem  that,  when  the  moisture  content  of  two  soils  is  near  the  satura- 
tion point,  they  will  be  in  equilibrium  at  moisture  values  measurable 
by  the  amount  which  either  soil  can  hold  against  the  force  of  gravity. 


Similarly,  at  much  lower  moisture  contents,  the  amounts  which 
the  two  soils  hold  against  a  force  one  hundred  or  one  thousand  tin 
as  great  as  gravity,  would  appear  to  establish  abasis  for  equilibrium. 
But,  in  view  of  the  fact  that  at  a  low-moisture  content  actual  capil- 
lary movement  becomes  negligible  while  transfer  from  one  to  t 
other  by  the  vapor-transfer  method  can  be  readily  accomplished,  it 
seems  more  logical  that  we  should  consider  an  equilibrium  existing 
which  would  mean  equal  osmotic  pressures  in  the  two  soils. 
points  can  be  determined  for  each  soil  by  freezing-] ><>mt  depressioi 
or  by  assuming  equal  osmotic  pressures  at  the  wilting  coefficients. 

Diagram  3  shows  a  method  for  working  out  a  scale  of  relations  fo 
the  soil  of  any  well  and  soil  from  three  depths,  obtained  when 
well  was  dug.     The  curve  for  the  well  soil  is  a  straight  line  whos 


76  BULLETIN   1059,   IT.    S.    DEPARTVlhXI     OF    AGBICULTUBE. 

ordinates  and  abscissae  are  equal.  It  need  not  be  drawn  at  all, 
except  for  illustrative  purposes.  For  each  of  the  native  soils  three 
or  more  points  may  be  plotted.  The  capillarity  point  lias  as  its 
ordinate  the  actual  capillarity  of  this  soil;  but  the  abscissa  has  the 
value  of  the  capillarity  for  the  well  soil,  similarly,  for  the  moisture 
equivalent  at  100  gravity  (which  is  still  a  capillarity  measure)  and 
the  wilting  coefficient,  or  any  other  point  at  which  osmotic  pressure 
of  the  several  soils  would  be  in  equilibrium.  (Sec  also  diagrams  8, 
9,  and  10  and  discussion,  p.  116.) 

These  curves  may  then  be  used  to  transpose  moisture  values  for 
the  soil  well  directly  into  moisture  values  for  the  native  soil,  realiz- 
ing the  probability  which  has  been  mentioned  that  at  any  moment 
the  well  and  native  soil  may  be  far  from  a  state  of  acttial  equilibrium. 

TECHNIQUE    OF    PERIODIC    SAMPLING. 

The  field  wrork  of  soil  sampling  is  essentially  the  same  where  soil 
wells  are  used  as  wxhere  it  is  feasible  to  sample  the  native  soil.  Soil 
samples  should  be  taken  at  permanent  stations  at  least  weekly  during 
the  open  season.  Definite  depths  of  1,  2,  and  3  feet,  and  more  if 
necessary,  wrill  recommend  themselves  in  preference  to  long  cores, 
which  showT  less  definitely  the  location  of  the  moisture.  The  1-foot 
sample  may  be  obtained  by  a  core  extending  from  10  to  14  inchi 
the  2-foot,  at  22  to  26  inches;  and  the  3-foot,  at  34  to  38  inches.  If 
intermediate  values  are  desired,  they  may  be  obtained  by  interpola- 
tion. 

Each  sample  as  obtained  should  be  placed  in  a  soil  can,  the  num- 
ber of  wdiich  may  immediately  be  entered  on  a  convenient  field 
form,10  together  with  the  number  of  the  station  and  I  he  depth.  There 
is  an  infinite  variety  of  soil  cans,  but  perhaps  the  most  generally  serv- 
iceable form  is  a  rather  heavy,  stamped  aluminum,  screwtop  can, 
about,  2\  inches  in  diameter  and  2\  inches  high. 

Soil  cans  containing  moist  soil  must  be  shielded  from  the  sun  and 
from  excessive  heat,  and  should  be  weighed  at  the  earliest  oppor- 
tunity, the  weight  being  ordinarily  determined  to  the  nearest  centi- 
gram.    (Fig.  2.) 

Soil  samples  of  any  ordinary  texture,  and  of  weight  not  exceeding 
100  grams,  should  be  dried  for  at  least  eight  hours  in  an  oven  having 
the  temperature  of  boiling  water.  Unusually  moist  samples,  or  thos< 
of  very  fine  texture,  should  be  given  a  longer  period.  Drying  for  24 
hours  is  not  too  long  to  make  good  results  certain.  Especial  care 
must  be  used  with  humus  soils  of  low  conductivity.  Only  trial 
weighing  will  show  wdien  a  sample  is  as  dry  as  possible  for  the  Condi- 
tions of  the  oven. 


»o  Forest  Service  Form  486,  fitting  notebook  cover  874-C. 


RESEARCH  METHODS  IX  STUDY  OF  FOREST  ENVIRONMENT. 


I   , 


After  drying,  each  soil  can  should  be  immediately  covered  and 


eiglied . 


The  moisture  percentage  should  be  determined  on  the  basis  of  dry 
weight  of  soil  by  deducting  the  can  weight  from  the  dry  weight, 
deducting  the  dry  weight  from  the  wet  weight,   and   dividing  the 


Fig.  2.— Ordinary  soil  cans,  foi  collection  of  moisture  samples,  with  covers  removed .    Size  J1,  by  -''  inches 

second  remainder  by  the  first  remainder.     Moisture  percentages  are 
usually  worked  out  to  one  or  two  decimal  places. 

Soil  cans  in  continuous  use  should  be  weighed  at  least  twice  each 
season,  preferably  at  the  beginning  and  middle  of  their  periods  of  u 
Aluminum  cans,  while  satisfactory  in  most  respects,  wear  appre- 
ciablv  in  a  few  months. 


78 


BULLETIN   1059,   U.    S.    DEPARTMENT   OF   AGRICULTURE. 


<o  Ph 


- 

g 

3 


at 


ft. 


P 

i 

fa 

CO 

o 

a 

"3 

Native 
soil  %. 

6 

-*> 

03 

- 

Sample 
(Tl)  %. 

P 
P 

^ 

CD 

H 

co 

'3 

a 

'3 

XII 

Native 
soil  %. 

Ratio. 

Sample 
(Tl)  %. 

c 

s 
P 

U 

u 

3 

CO 

•3 

a 

'3 

EG 

Native 
soil  %. 

Ratio. 

Sample 

(Tl)  %. 

X- 

c 

a 
P 

>< 

o5 

fa 
+j 

CO 

'3 

a 

o 

GO 

Native 
soil  %. 

6 

eg 

Sample 
(Tl)  %. 

a 

> 

> 

CD 

a 

CO 

o 

a 

o 

GO 

Native 
soil  %. 

— 

|— 

6 

03 

Sample 

(Tl)  %. 

CD 

eS 
P 

- 
— 

c 

E 
-7 

i 

i 
> 

*. 
1 
£ 

c 

r 
0. 

i 

I 

> 

O 
PI 

< 

w 

fa 

P 

s 

c 

fa 

fa 
W 

K 

P 
fa 

y; 

»™> 

Z 

= 

o 

DQ 

fa 

C 

tl 
PS 

< 

> 

< 

1 

fa 
o 

oo 
Z 

fa 

fa 

P 

fa 

c 
* 

; 

• 

• 

i 

It 

■ 
; 

V 

■a 

ft 

5 

a 

c 
5 

B 

3 

X 

- 

4- 
C 

: 

E 

c 
c 

) 

> 

M 

Z 

i  - 
i  s 

e  = 

- 

7 

I    - 

- 

— 
—  3 
3     - 

&     = 


z 
- 

— 
— 

-- 

- 

- 

* 


z 
= 


c 

— 
M 
- 


RESEARCH  METHODS  IN  STUDY  OF  FOREST  EX  VI  ,;<  ,N  M  i.x  i  .         79 

The  figures  resulting  from  the  above  computations  for  the  tnoi 
ture  of  the  soil  in  soil  wells  may  be  tabulated  on  the     -    ,1  Moisl  1 
form  in  the  columns  headed  "Sample,  Tl."     The  ratio  which  has 
been  determined  between  the  moisture  of  the  soil  in  the  well  and  thai 
of  the  native  soil  at  same  depth,  and  the  computed  moisture  of  the 
surrounding  soil,  or  the  moisture  figure  read  directly  from  curvi 
may  also  be  entered  for  each  date.     If  the  native  soil  moisture  is 
directly  determined  by  sampling,  only  the  third  column  under  each 
depth  will  be  used.     Space  is  also  provided  on  the  "Soil  Moisture" 
form  for  any  computations  which  it  is  desired  to  make  either  cur- 
rently or  after  obtaining  the  monthly  means;  such  as,  for  example, 
the  percentage  of  available  moisture,  the  availability,  or  the  various 
percentages  on  a  volume  basis.     Appropriate  headings  may  be  sup- 
plied. 

Determination  of  Nonavailable  Moisture. 

The  method  of  soil  wells  does  not  attempt  to  standardize  soils  for 
different  localities,  which  could  only  be  done  thorough  Iv  by  using 
soil  from  one  source  in  all  soil  wells.     Nor  is  it  desirable  that  soils 
different  localities  should  be  compared  on  the  same  physical  bas 
since  this  physical  basis  of  itself  determines  quite  largely  the  mean 
water  content  of  the  soil  and  its  attraction  for  a  given  species.     It  is, 
however,  necessary  before  different  sites  and  localities  may  be  satis- 
factorily compared  as  to  their  soil  moisture  that  it  should  be  known, 
at  least  approximately,  at  what  points  they  become  physiologically 
dry,  either  for  plants  in  general  or  for  plants  of  a  given  specii 
Briggs  and  Shantz  (114),  it  is  true,  after  an  exhaustive  study  of  tin- 
subject  which  has  cleared  the  way  for  many  other  investigations, 
summarize  in  part  as  follows: 

The  results  of  this  investigation  have  led  us  to  conclude  that  the  differences  ex- 
hibited by  plants  in  this  respect  are  much  less  than  have  heretofore  been  supposed, 
and  are  so  small  as  to  be  of  little  practical  utility  from  the  standpoint  of  drought 
sistance.  As  compared  with  the  great  range  in  the  wilting  coefficienl  due  to  soil 
texture,  the  small  differences  arising  from  the  use  of  different  species  of  plants  in 
determining  the  wilting  coefficient  become  almost  insignificant. 

Expressing  this  difference  numerically,  it  is  said: 

Taking  100  to  represent  the  average  wilting  coefficient,  the  differenl  Bpe<  sted 

(except  Colocasia  and  Isoetes)  give  an  extreme  range  from  92  for  Japan  rice  to  I 
a  variety  of  corn. 

From  these  experiments  and  conclusions  the  impression  has  grown 
up  that  all  plants  are  capable  of  extracting  the  moisture  of  the  Boil 
to  essentially  the  same  basic  point.     Shantz  may  be  quoted  as 
ing  that  there  was  no  intent  to  convey  this  impression,  and  expei 
ments  to  be  described  later  will  show  that  as  between  tree  species 


80  BULLETIN   1059,  U.   S.   DEPARTMENT  OF  AGRICULTURE. 

adapted  to  radically  different  habitats,  there  may  be,  at  least  under 
certain  conditions  of  wilting,  radical  differences  in  the  coefficients. 
Another  important  phase  of  the  matter  is  that  certain  soils  may 
have  a  peculiar  reaction  on  one  species  and  not  on  others;  as,  for 
example,  a  highly  acid  or  strongly  limey  soil.  It  is  therefore  th< 
part  of  wisdom  to  test  the  nonavailable  moisture  of  any  soil  by  the 
use  of  at  least  the  predominating  or  type  species  found  on  the  soil, 
and  of  as  many  other  species  as  possible. 

DIRECT  DETERMINATION  OF  THE  WILTING  COEFFICIENT. 

The  writers  cited  above  have  given  such  thorough  consideration 
to  this  and  the  succeeding  subjects  that  a  complete  discussion  here 
appears  almost  useless.  The  treatment  of  forest  soils  and  forest 
species,  however,  has  brought  out  a  number  of  new  problems,  so  that 
it  is  almost  impossible .  to  overlook  any  phase  of  the  question  in 
this  discussion.  Constant  comparison  will  be  made  with  the  treat- 
ment found  desirable  for  field  crops  and  related  plants. 

It  is  well  to  bear  in  mind  from  the  outset  the  point  brought  out 
by  Briggs  and  Shantz  that  the  wilting  coefficient  represents  merely 
the  moisture  point  at  which  wilting  first  occurs  to  such  an  extent 
that  the  plant  does  not  recover  if  placed  in  a  saturated  atmosphere. 
The  plant  may  actually  draw  considerable  water  from  the  soil  after 
this,  and  might  be  theoretically  conceived  to  pass  moisture  to  the 
atmosphere  until  the  soil  and  atmosphere  were  in  vapor-pressure 
equilibrium.  The  wilting  coefficient  is,  however,  the  practical  ex- 
pression for  nonavailable  moisture. 

The  fact  must  be  also  strongly  emphasized  that  the  point  at  which 
wilting  occurs  must  depend  in  a  very  large  measure  on  the  rate  at 
which  the  plant  is  transpiring;  or,  in  other  words,  on  atmospheric 
conditions  and  sunlight.  Therefore,  as  the  soils  approach  dryness, 
the  conditions  should  be  maintained  at  a  fairly  definite  standard. 
It  will  usually  be  feasible  to  prevent  the  occurrence  of  temperatures 
in  excess  of  70°  F.,  as  well  as  sudden  changes  in  temperature,  and 
to  exclude  direct  sunlight.  It  would  be  also  desirable  to  control 
atmospheric  moisture,  though  this  is  a  very  difficult  thing  to  do  in 
ordinary  rooms. 

In  the  tests  with  seedlings  of  coniferous  trees  it  has  been  found 
exceedingly  difficult  to  determine  when  permanent  wilting  occurs. 
There  is  no  doubt  that  seedlings  of  this  kind  have  developed  a  power 
of  resistance,  or  recovery,  far  in  excess  of  that  of  most  plants.  This 
probably  consists  in  an  extremely  low  rate  of  transpiration  when 
the  moisture  becomes  deficient;  but  the  difficulty  of  observation  may 
be  mainly  ascribed  to  the  fact  that  the  stems,  and  to  a  Lesser  extent 
the  leaves,  become  stiff  and  woody  at  a  very  early  age,  so  thai 
shriveling  rather  than  collapse  is  the  phenomenon  that   evidences 


RESEARCH  METHODS  IN  STUDY  OF  FOREST  ENVIRONMENT.         8  1 

lack  of  water  in  the  aerial  portions.     This  is  particular] 
yellow  pine  and  Douglas  fir  seedlings,  at  the  age  of  six  w<  r  moi 

while  lodgepole  pine  does  not  harden  until  much  later. 

For  these  reasons  wilting  can  rarely  be  recorded  in  a  large  Dum- 
ber of  seedlings  simultaneously,  and  it  is  therefore  desirable  that 
the  moisture  content  should  be  recorded  as  each  seedling  wilts,  an 
algebraic  mean  content  being  computed  when  the  process  ls  i iplel 

While  Briggs  and  Shantz  found  it  desirable  to  grow  thi 
in  small  glass  pots  (these  seem  to  have  had  about  the  dimensions  of 
drinking  glasses),  the  heterogeneous  character  of  nearly  all   for< 
soils  necessitates  the  treatment  of  samples  large  enough  to  include 
a  normal  proportion  of  rock  fragments.     If  these  are  very   large 
they  may  be  broken  down  to  maximum  dimensions  of  about  2  inch 
without  appreciable  alteration  of  their  relations  to  moisture,   but 


'<*&*& 


Fig.  3.— Echard  pans,  7  by  3  inches,  containing  seedlings. 

that  is  all  that  can  be  done.  These  rocks  can  not  be  eliminated 
altogether,  as  it  is  found  that  the  more  permeable  of  them  may  hold 
1  to  2  per  cent  of  nonavailable  moisture.  They  are  distinctly  a 
part  of  the  soil,  and  it  is  their  presence  which,  in  a  large  measure, 
makes  the  soil  capable  of  supporting  forest  growth. 

A  pan  (fig.  3)  which  meets  these  requirements  is  made  oi  20- 
gauge  or  24-gauge  galvanized  iron,  3  inches  deep  and  7  inches  in 
diameter.  Half  a  dozen  small  perforations  in  the  flat  bottom  permit 
drainage  while  the  seedlings  are  being  started,  and  aeration  at 
the  surface  of  the  soil  has  been  sealed  over.  When  filled  to  a  depth 
of  2£  inches,  such  pans  hold  3  to  6  pounds  of  soil. 

To  avoid  any  chemical  change  in  the  soil,  but  more  partici 
to  keep  it  receptive  toward  moisture,  the  dry  weight  of 
in  the  pan  is  determined  rather  by  moisture  samples  secured 
pan  is  being  filled  than  by  drying  the  whole  ma 
over,  is  a  slow  process,  especially  with  soils  of  low  conductivity. 

10163— 22— Bull.  1059 6 


82  BULLETIN   1059,   U.    S.    DEPARTMENT  OF   AGRICULTTJBE, 

However,  no  bad  effects  whatever  have  been  noted  from  drying 
fairly  sandy  soils  at  the  standard  temperature. 

After  obtaining  the  air-dry  weight  of  the  pan  and  soil,  an  amount 
is  removed  from  the  pan  sufficient  to  form  a  layer  one-fourth  of 
an  inch  deep.  The  coarser  material  is  excluded  from  this  lot,  which 
is  to  form  a  covering  for  the  seeds.  With  this  taken  out,  the  re- 
maining soil  is  leveled  down  with  a  spoon,  the  seeds  are  sown  on 
this  smooth  surface,  and  the  covering  soil  is  replaced.  i 

The  number  of  seeds  to  be  sown  should  be  gauged  according  to 
known  viability,  so  as  to  produce  about  100  seedlings  in  a  pan  of  this 
size.  The  weight  of  the  seeds  is  obtained  before  sowing,  and  this 
weight  is  considered  throughout  as  an  addition  to  the  tare.  The 
further  assumption  is  made  that  the  weight  of  the  seedlings  will  not 
at  any  time  appreciably  exceed  the  weight  of  the  seeds. 

Having  calculated  the  net  dry  weight  of  the  soil  from  the  mois- 
ture content  of  the  dried  sample,  the  moisture  content  of  the  soil  at 
any  stage  in  development  or  wilting  of  the  plants  is  calculated,  after 
a  weighing  of  the  pan,  by  the  equation: 

W—  (Pan,  seed,  soil,  and  paraffin) 
Moisture  percentage  equals  100  X g^ —  ■ 

The  pans  are  placed  in  a  greenhouse  where  they  may  have  the 
necessary  light  and  warmth  to  induce  prompt  germination,  and  for 
the  sake  of  uniform  development  and  conditions  affecting  wilting 
are  preferably  kept  on  a  revolving  table. 

The  soils  are  watered  exclusively  with  distilled  water,  both  to  avoid 
the  introduction  of  spores  and  the  addition  of  salts,  which,  in  the 
absence  of  drainage,  might  appreciably  increase  the  wilting  coefficient . 
Nothing  suggestive  of  a  toxic  effect  from  this  distilled  water  has  been 
noted.  It  is  desirable  to  aerate  the  water  as  much  as  possible  before 
applying.  Under  ordinary  atmospheric  conditions,  the  pans  will 
require  50  to  60  cubic  centimeters  per  day  to  maintain  moisture 
favorable  for  germination. 

In  working  with  deeper  mineral  soils,  damping  off  of  seedlings  is 
rarely  noted,  but  surface  soils  from  the  forest  often  contain  the 
damping-off  fungi.  In  fact,  this  is  so  common  that  many  observa- 
tions which  ascribed  the  death  of  seedlings  in  the  forest  to  unfavor- 
able physical  conditions  may  be  questioned.  Certain  it  is  that 
damping  off  in  the  wilting  pans  may  cause  the  greatest  confusion, 
if  they  do  not  actually  vitiate  the  tests.  Soils  suspected  of  contain- 
ing these  organisms  should  therefore  be  treated,  several  days  before 
the  seed  is  sown,  with  a  solution  of  formaldehyde,  as  suggested  by 
Hartley  (124)  for  nursery  beds.  This  should  be  used  at  the  rate  of 
about  one-eighth  of  a  fluid  ounce  per  pan,  dissolved  in  sufficient  clean 
water  to  reach  all  soil  in  the  pan.     Opportunity  should  afterwards 


RESEARCH  METHODS  IN  STUDY  OF  FOREST  ENVIRONMENT. 

be  given  for   the  formaldehyde   to   evaporate  entirely.     This   will 
doubtless  occur  before  the  soil  is  perfectly  dry. 

When  germination  is  fairly  complete,  the  seedlings  well  established 
so  as  to  reach  all  parts  of  the  soil,  and  the  tendency  to  succumb  bo 
damping  off,  if  any,  outgrown,  the  surfaces  of  the  puns  are  sealed 
over  by  pouring  on  the  top  of  the  soil,  previously  Leveled,  about  50 
grams  of  a  melted  mixture  of  paraffin  and  petrolatum  (veterinary 
vaseline  is  one  of  the  least  expensive  forms)  in  the  proportion  of  2:1. 
This  congeals  at  40°  C.  and  may  be  applied  at  50°  C.  without  anj 
injury  to  the  stems  of  the  seedlings.  Not  infrequently,  if  the  XN  ilting 
process  requires  many  days,  the  seal  will  draw  away  from  the  edgi 
of  the  pan,  but  this  is  easily  rectified  by  the  use  of  a  blunt,  smooth 
stick.  At  any  rate,  it  is  not  essential  absolutely  to  prevent  direct 
evaporation  from  the  soil,  though  a  more  even  distribution  of  moisture 
may  be  expected  if  such  loss  is  kept  at  a  minimum. 

The  weight  of  paraffin  added  is  determined  by  weighing  the  beaker 
from  which  it  is  poured  before  and  after  each  application.  This 
makes  a  further  addition  to  the  tare.  The  soil  should  be  fairly  moist 
when  the  paraffin  is  applied,  so  that  the  latter  will  not  penetrate. 

With  coniferous  seedlings,  provided  a  good  stand  has  been  secured, 
the  withdrawal  of  moisture  and  the  sealing  of  the  pans  may  usually 
be  undertaken  at  four  to  six  weeks  after  sowing;  though  in  the  case 
of  spruce  and  perhaps  other  species  which  root  rather  slowly  a  slightly 
longer  period  maybe  desirable.    As  has  been  pointed  out,  the  diflieultv 
of  detecting  wilting  increases  as  the  seedlings  become  older  and  more 
completely  lignified.     It  is  also  unmistakably  true  that   the  older 
the  seedling  the  more  difficult  it  is  to  kill.     This  is  probably  due  in 
part  to  greater  resistance  to  drying  out  and  in  part  to  deeper  or  more 
extensive  rooting,  which  would  be  an  advantage  if  the  moistuTe  at, 
say,  the  bottom  of  the  pan,  were  not  being  drawn  on  as  freely   as 
that  near  the  surface.     However,  observations  on  the  wilting  oi  seed- 
lings under  direct  insolation  point  unmistakably  to  resistance 
creasing  with  age.     When  the  surface  of  the  soil  becomes  extremely 
warm,  even  if  there  is  an  abundance  of  moisture  within  reach  oi 
roots,  wilting  is  likely  to  be  evidenced  by  collapse  oi  the  stem  at 
ground  line.     The  phenomenon  is  almost   identical  when  the 
face  of  the  soil  becomes  dry  in  advance  of  the  deeper  soil.      I  he  se 
ling  is  undoubtedly  vulnerable  to  water  loss  and  critical  injury 
the  lower  part  of  the  stem.     Under  such  conditions  ,t  is  noted  tha 
the  younger  seedlings  usually  succumb  first,  and  those  which  survive 
one  exposure  are  killed  by  a  repetition  which  is  still  more  se. . 

It  is  evident,  therefore,  that  age  of  seedlings  will  have  an    ... poi  - 
ant  influence  on  the  results,  though  this  will  not  be  so  important 


84  BULLETIN   1059,   U.    S.    DEPARTMENT  OF   AGRICULTURE. 

the  test  is  conducted  in  such  manner  as  to  keep  the  moisture  uniform 
throughout  the  soil,  and  hence  uniform  for  the  deepest  and  shallow- 
est-rooted seedlings. 

It  is  also  self-evident  that  specific  differences  may  be  brought  out 
by  one  set  of  conditions,  which  would  not  be  apparent  in  another 
set,  particularly  conditions  which  make  the  requirement  for  moisture 
great  or  small.  If  transpiration  is  very  rapid,  seedlings  of  a  shallow- 
rooted  species  may  be  unable  to  meet  this  demand,  while  deeper- 
rooted  seedlings  in  the  same  pan  may  pull  through,  because  their 
supply  at  this  stage  is  somewhat  more  readily  obtained.  For  an 
actual  test  of  drought  resistance,  therefore,  it  is  fundamentally 
necessary  that  the  transportation  and  soil-drying  process  should  be 
slow  enough  to  permit  equalization  of  the  opportunities  before  the 
critical  test  comes.  Hence  the  standard  conditions  of  exposure 
which  have  already  been  suggested. 

The  method  of  recording  the  death  of  each  seedling  in  a  lot  of  100, 
together  with  the  pan  weight  and  calculated  moisture  accompanying 
such  death,  has  a  distinct  advantage  over  the  method  which  permit- 
only  one  determination  of  the  moisture  content  when  all  of  the 
seedlings,  or  a  majority  of  them,  have  succumbed.  It  gives  an  indi- 
cation of  the  possible  variation  between  individuals  of  the  same 
species,  and  a  measure  of  the  probable  experimental  error  due  both 
to  this  variation  and  to  uneven  distribution  of  moisture  in  the  pan. 
which  is  not  wholly  unavoidable.  What  it  really  amounts  to  is 
practically  100  separate  tests  on  100  sections  of  soil.  If,  on  the  one 
hand,  the  first  losses  occur  in  sections  of  the  soil  which  have  unavoid- 
ably become  drier  than  the  average,  on  the  other  hand,  the  last  sur- 
vivors are  undoubtedly  in  areas  which  are  at  the  opposite  extreme. 
These  variations  should  be  largely  compensated  by  taking  the  alge- 
braic mean  of  all  the  moisture  determinations,  a  figure  in  which  a 
great  deal  of  confidence  can  be  placed. 

INDIRECT   METHODS    FOR    WILTING    COEFFICIENTS. 

Inasmuch  as  the  direct  determination  of  the  wilting  coefficient  is 
a  process  which  is  likely  to  require  several  wTeeks,  at  the  best  is  liable 
to  rather  large  experimental  errors,  and  is  also,  without  question,  in- 
fluenced by  the  kind  of  plant  used,  various  methods  have  been  devel- 
oped by  which  the  affinity  of  the  soil  for  water  may  be  determined; 
and  the  amount  of  water  held  by  it  under  certain  empiric  conditions 
of  the  test  may  be  related  to  the  amount  which  would  be  held  against 
the  pull  of  plants. 

In  addition  to  furnishing  a  ready,  if  only  approximate,  index  to 
the  soil  conditions  which  may  be  encountered  in  the  field,  and  espe- 
cially an  index  to  the  danger  of  early  drought,  it  seems  that  the  use 
of  indirect  methods,  employing  definite  physical  forces  for  the  crea- 


RESEARCH  METHODS  IN  STUDY  OF  FOREST  EXYIKOX.v 

tion  of  a  certain  condition  of  soil  moisture,  has  entifi< 

which  fully  justifies  an  elaborate  description  of  them.     For  examp 
such  methods  permit  us  to  compare  the  drought  resistance  of  eu 
number  of  species  in  any  number  of  soils  through  any  period,  pi 
vided  only  that  the  experimental  conditions  are  reproducible.     \\ 
can  determine  this  relative  drought  resistance,  as  between  two  or 
three  species,  by  wilting  them  simultaneously  in  the  same  soil  mas 
and  gradually,  by  one  comparison  and  another,  include  all  of  our 
species  and  all  of  our  soils.     Even  this  method,  however,  is  not  free 
from  the  necessity  for  uniform  conditions  in  the  successive  tests.     It 
is  therefore  best  that  each  wilting  coefficient,  while  being  determined 
under  some  arbitrary  and  standard  set  of  conditions,  should  be  n 
lated  to  some  other  measure  of  the  soils'  water-holding  capacity 
which,  under  reproducible  test  conditions,  always  means  just   one 
thing.     In  this  way  an  enormous  number  of  comparisons  may  be 
made  between  the  wilting  coefficients  for  different  soils  and  different 
species.     Such  physical  determinations  may  also  lead  to  a  critical  ex- 
amination of  wilting  coefficients  and  to  the  most  desirable  standard 
methods  for  their  determination. 

Of  the  various  indirect  methods  which  have  been  devised  may  l>e 
mentioned: 

1.  The  determination  of  the  antiosmotic  pressure  of  the  soil,  corre- 
sponding to  the  maximum  osmotic  pressure  which  the  species  under 
consideration  is  known  to  tolerate  without  fatal  results.  This  method 
is  obviously  not  so  useful  as  the  others,  since  it  presupp^c-  some 
knowledge  of  the  plants  which  may  not  be  available.  It  must  ae< 
sarily  consist  of  a  number  of  determinations  on  the  same  kind 
soil,  at  different  moisture  contents,  until  the  moisture  condition  is 
found  at  which  the  freezing  point  becomes  "submerged";  that  is, 
becomes  indeterminate.  Obviously,  this  leads  to  the  region  in  which 
the  freezing-point  determinations  are  least  precise.  While  not 
abandoned,  this  method  will  be  laid  aside  to  be  discussed  more  fully, 
and  in  its  most  useful  aspects,  in  connection  with  the  coefficient  ol 

availability. 

2.  The  capillary  moisture  determination,  in  which  the  soil  is  allowed 

to  demonstrate  its  ability  to  hold  water  against  the  tone  oi  gravit 

3.  The  moisture-equivalent  determination,  in  which  the  mois 

in  the  soil  is  subjected  to  any  definite  force,  dependent  on  its  owl 
mass.     This  may  be  a  force  created  by  the  centrifugal  method  one 
hundred  or  one  thousand  times  as  great  as  gravity. 

4.  The  hygroscopic  coefficient  determination,  m  which  the  at 

of  the  soil  for  moisture  is  determined  by  exposing  it  to  an  atmospn* 

of  saturated  vapor.  .  , 

The  capillary  moisture,   or  '•capillarity,"   the   terms  being   used 

interchangeably,  in  this  discussion,  refers  to  the  quantity  ol  watei 


86  BULLETIN    1059,   U.    S.    DEPARTMENT    OE    AGRICULTURE. 

that  may  be  held  by  the  soil  against  the  force  of  gravity.  This 
amount  decreases  as  the  height  of  the  column  of  soil  increases,  and 
may  also  be  considerably  influenced  by  the  packing  of  the  soil. 
Since  it  is  almost  impossible  to  treat  any  soil  in  the  same  state  of 
compactness  in  which  it  is  found  in  the  field,  or  to  establish  a  stand- 
ard condition  for  soils- in  vessels,  this  measure  of  the  water-holding 
power  of  a  soil  is  not  likely  to  have  precise  value.  The  greatest 
theoretical  objection  to  it  is,  that  the  force  tending  to  remove  the 
water  from  the  soil  is  of  an  entirely  different  magnitude  from  thai 
at  work  as  the  plant  makes  its  final  struggle  for  water,  and  that 
the  effect  produced  by  the  one  force  can  not  serve  as  a  measure  of 
the  effect  which  might  be  produced  by  the  other.  There  seems  also 
to  be  an  impression  that  salts  in  the  soil  water  operate  to  raise  the 
wilting  coefficient,  while  decreasing  the  capillary  moisture  by  lower- 
ing the  surface  tension  of  the  liquid.  Such  an  impression  arises 
from  the  well-known  effect  of  foreign  substances  on  the  surface  of  a 
liquid.  It  has  been  pointed  out  by  Free  (121)  that  salts  in  solution 
actually  increase  the  surface  tension  of  the  liquid,  and  this  is  entirely 
in  keeping  with  the  known  properties  of  solution-.  While  the  pres- 
ence of  solutes  may  have  the  effect  of  weakening  the  affinity  of  one 
water  molecule  for  another,  this  is  fully  counterbalanced,  in  its  rela- 
tion to  capillarity,  by  the  greater  density  of  each  group  of  molecules 
of  which  the  solute  forms  a  nucleus,  and  the  consequent  greater 
affinity  between  such  groups  and  the  solid  surface.  This  affinity  i- 
known  by  the  name  of  '•capillary  attraction."  Furthermore,  even 
while  admitting  that  in  either  the  capillary-moisture  test  or  the 
moisture-equivalent  test  some  of  the  solutes  may  he  lost  with  the 
water  which  is  drained  out  of  the  soil,  considerable  satisfaction  is 
gained  from  the  idea  previously  set  forth  that,  at  the  wilting  point 
of  soils,  these  solutes  may  be  absorbed  by  the  colloids. 

It  is  believed  that  Hilgard  (125)  was  the  first  to  employ  the 
principle  of  capillarity  for  comparing  soils.  He  used  a  sieve  cylin- 
der only  1  centimeter  high,  which,  after  a  layer  of  filter  paper  was 
placed  in  the  bottom,  was  filled  level  with  the  soil.  This  was  im- 
mersed to  a  depth  of  1  millimeter  in  distilled  water,  allowed  to  stand 
for  an  hour,  and  then  weighed.  The  amount  of  water  absorbed,  of 
course,  was  dependent  on  the  ability  of  the  soil  to  lift  it.  a  maximum 
distance  of  9  millimeters. 

Briggs  and  Shantz  (114)  compared  this  measure  of  absorbing 
capacity  with  the  directly  determined  wilting  coefficients  of  15  -oil- 
whose  wilting  coefficients  ranged  form  0.9  to  16.7  per  cent.  From 
these  comparisons  it  is  evident  that  a  soil  which  is  able  to  withhold 
almost  no  moisture  from  plants  has  a  fairly  high  capillarity,  but 
that  the  latter  does  not  increase  in  so  great  a  proportion  as  the 
wilting  coefficient  with  more  retentive  soils.     Thus  it   was  neces- 


RESEARCH  METHODS  IN  STUDY  OF  FOREST  ENVIRONMENT. 

sary  to  subtract  21  from  the  percentage  of  capillary  moisture 
obtain  a  quantity  having  a  fairly  constant  ratio  to  the  wilting 
efficient. 

r™  .    •       •         r     Capillarity  — 21     „ 

This  is  given  by  Wiltin(y  Qoeff   =  2.90 ± 0.06,  or±2.1  per  cent. 

The  probable  error  of  a  single  determination  by  this  means  was 
found  to  be  8.3  per  cent  of  the  wilting  coefficient. 

In  the  treatment  of  forest  soils  Bates  (105),  at  the  Fremont  Ex- 
periment Station,  has  found  it  necessary  to  use  much  larger  cans 
than  those  employed  by  Hilgard,  and  has  also  reversed  the  pro< 
so  that  the  result  is  rather  a  measure  of  the  ability  <«f  the  soil  to 
hold  the  water-  of  saturation  than  to  lift  water  from  below.  A  gal- 
vanized can  5§  inches  deep  and  4  inches  in  diameter,  is  filled  to  a 
depth  of  5  inches  with  air-dried  soil,  which  is  jarred  and  tampered 
until  no  appreciable  settling  occurs.  This  can  is  perforated  in  the 
bottom  and  a  filter  paper  is  used  to  keep  the  soil  from  sifting  out. 
The  can  is  immersed  to  its  full  depth  in  water,  but  no  water  is  al- 
lowed to  flow  on  the  top  of  the  soil.  As  the  water  rises  from  the 
bottom  by  its  own  pressure,  the  air  is  pushed  out,  so  that  lew.  if 
any,  air  spaces  are  left.  The  samples  are  allowed  to  soak  at  Last 
24  hours  to  insure  complete  absorption  by  the  larger,  permeahle  rock 

fragments. 

The  weight  attained  at  the  end  of  this  period,  or  a  longer  period 
if  it  appears  necessary,  is  an  index  to  the  saturation  capacity. 

The  cans  are  now  placed  on  a  drain  board,  covered,  and  allowed  to 
stand  for  48  hours.  In  rehandling  the  cans  care  must  be  used  to 
avoid  jarring,  as  some  of  the  water  is  held  in  a  very  delicate  balance. 
The  amount  of  water  held  at  this  time  is  a  measure  of  the  capillary 
moisture.  In  the  vast  majority  of  soils  that  have  been  treated,  the 
capillary  moisture  is  about  90  per  cent  of  the  saturation  capacity. 
Clay  does  not  affect  this  ratio  appreciably,  but  humus  increases 

The  same  cans  are  now  used  for  the  centrifugal  test  or  moisture 
equivalent  determination,  which  will  shortly  be  described.     After 
they  are  oven-dried,  to  give  the  basis  for  dry-weighl   calculation* 
The  apparent  density  is  also  computed  from  the  weight  and  volume 

after  this  treatment. 

In  Table  2,  there  is  presented  a  comparison  of  the  capillary  moi 
tures  and  wdting  coefficients  of  10  soils  of  one  general  type    granil 
from  an  Engelmann  spruce  forest,  bnt  varying  widely  m  state  rt 
decomposition,  clay  content,  and  humus  content.     Each  aoU  repre- 
sents a  sample  extending  from  the  surface  to  a  depth  of  I  l-  • 
wilting  coefficients  for  Douglas  fir  and  Engelmann  f™- 
carefully  determined,   the  only  objection   that   nngh       e     ■ 
against  the  treatment  being  that  the  seedlings  were  given  n b» 


88 


BULLETIN   1059,  U.   S.   DEPARTMENT  OF   AGRICULTURE. 


light  than  now  seems  desirable,  although  not  enough  to  develop  ex- 
cessive temperatures.  In  contrast,  there  are  also  presented  the  re- 
sults of  nine  tests  on  coarse  granitic  gravels,  from  depths  of  1  to  3 
feet,  containing  minimum  amounts  of  humus  and  clay.  The  wilting 
coefficients  were  determined  in  the  same  manner  as  the  other  group, 
and  at  almost  the  same  time. 

Table  2. — Capillary  moisture,  moisture  equivalent,  and  wilting  coefficient  of  19  soils. 

(Wilting  coefficients  determined  synchronously.) 


Capil- 
lary 
Sample  No.     meas- 
ure 
(G). 


539. 
555. 
534. 
547. 
545. 
549. 
544. 
526. 
546. 
538. 


Per 

cent. 
17.07 
20.29 
24.06 
29.34 
33.77 
36.16 
41.56 
49.92 
60.00 
89.05 


Mean 

Mois- 

wilting 

ture 

coeffi- 

Humus 

equiva- 
lent 

cient 

by  ig- 

(spruce 

nition. 

(100-G). 

and 

fir).i 

Clay. 


Per 

cent. 
11.68 
11.72 
14.  45 
20.32 
22.02 
19.95 
26.30 
29.84 
42.72 
73.50 


Per 

cent. 
3.17 
3.00 
3.90 
6.01 
6.02 
7.04 
8.68 

S.til) 

20.46 
21.71 


Per 

cent. 

3.29 

3.52 

3.00 

6.56 

7.09 

6.04 

11.64 

9.75 

21.10 

27.00 


Per 

cent. 
4.5 
0.4 
2.2 
6.7 
3.6 
4.2 
3.4 
3.2 
2.  6 
2.7 


Water  2 
soluble 
matter. 


Ratio  mean 
wilting  coeffi- 
cient . 


To 
capil- 
larity. 


Parts 
per 
million. 
305 
275 
165 
270 
845 

1,030 
.530 
200 

1,250 


a  w. 
.us 

.  162 
.205 
.17s 
.  196 
.209 
.172 
.341 
.244 


To 
mois- 
ture 
equiva- 
lent. 


Ratio  final 
willing  coeffi- 
cient. 


Final3 
wilting 
coeffi- 
cient . 


To 
capil- 
larity. 


0.271 
.256 
.270 
.  296 
.273 
.353 
.330 

.479 
.295 


Per 

cent. 

2.56 

2.52 

3.  30 

5.26 

5.18 

6.10 

7.  83 

6.95 

11.17 

13.  89 


Group  averages,  granitic  loams. 


204 


,311 


0.150 
.  L24 
.137 
.179 
.1.53 
.169 
.  L88 
.139 
.  L86 
.156 


To 
mois- 
ture 
equiva- 
lent. 


.  158 


o.  219 
.215 
.228 
.259 
.235 
.306 
.298 
.233 
.  282 
.  189 

.214 


292. 
37.. 
235. 
36.. 
293. 
82.. 
83.. 
85.. 
84.. 


11.02 

5.04 

2.  73 

1.02 

1.5 

11.46 

3.53 

2.00 

1.  33 

0.2 

12.00 

5.06 

3.09 

2.09 

3.1 

12.95 

4.35 

2.47 

1.71 

2  7 

12.96 

5.62 

2.93 

1.79 

L5 

13.18 

4.86 

2.27 

1.S5 

1.9 

13.61 

5.19 

2.76 

1.76 

2.5 

15.30 

5.57 

2.54 

1.92 

2.  1 

15.65 

5.03 

2.54 

1.31 

3.7 

Group  averages,  granitic  gravels 200 

Grand  averages .415 

Mean  variation  of  single  values 13       .1157 

Percentage  of  mean  variation 16.  5 

Probable  error  in  average .  U230 

1  In  these  tests  spruce  and  fir  gave  almost  the  same  figures,  on  the  average. 

2  200  grams  soil  leached  on  filter,  with  1  liter  water,  through  24-hour  period.    For  the  gravel  group  re- 
sults are  approximations,  on  account  of  lack  of  sensitive  scale. 

3  Average  of  the  moistures  existing  for  last  tree  of  each  species. 

To  avoid  duplication  of  tables  later  there  are  also  inserted  here 
the  moisture  equivalents  of  the  same  soils. 

The  comparison  of  capillary  moisture  and  wilting  coefficients 
given  in  Table  2  brings  out  the  following  facts: 

1.  An  examination  of  the  column  headed  u  Ratio  of  mean  wilting 
coefficient  to  capillarity "  shows  that  there  is  considerable  variation 
in  the  individual  results.  In  the  first  group  the  two  results  which 
are  appreciably  higher  than  the  average  are  those  for  samples  of  the 
highest  capillarity,  resulting  from  unusual  quantities  of  humus. 


RESEARCH  METHODS  IN  STUDY  OF  FOREST  ENVIRONMENT, 

2.  The  group  averages  for  the  loamy  spruce  soils  and  the  granil 
gravels  are  essentially  the  same.  It  therefore  seems  entirely  legiti- 
mate to  consider  both  groups  together  and,  as  shown  m  diagram  1. 
to  express  the  relation  of  wilting  coefficient  to  capillarity  l>\  a  si  raighl 
line.  The  nineteen  cases  show  an  average  variation  of  0.0333  from 
the  mean  ratio  of  0.202,  or  16.5  per  cent  variation. 

3.  Both  from  its  mean  value  and  from  the  facl  thai  the  graph 
which  expresses  this  relationship  passes  through  the  main  axis  of 
the  system  of  coordinates  it  is  evident  that  the  capillary  moisture 

it  has  been  measured  by  the  method  described  above  is  an  entirely 


DIAGRAM   4 

RELATION      OF 

MEAN   WILTING   COEFFICIENT  TO  CAPILLARY   MOISTURE 

IN   10    SPRUCE    SOILS    AND   9   GRAVELS 

LEGEND 

2.75    Humus     percen+ate 
O     SPRUCE    GRANITIC    LOAMS 

650  Solutes,ppm  of  soil  wt. 
o      GRANITIC    GRAVELS 

?n                                                                                                                                                     ■ 

no 

?,» 

C 

i 

> 
00 

o 

L 

i*S 

+? 

U 

, 

o 

u 

i 

H64 

975 
O 

sin 

a  — — 

5 

S>-J£^ 

so 
■^of 

55 

.£.  C 
TO 
V 

,  2 

^\ 

10 

-T-10 

M5 

L 

•^ 1 

it        4 

1 

n 

t 

■ 

- 

1 

e       2 

0 

£ 

g        : 

C 

iZ          3 

jpiH 

2         ' 

?ry 

rtoist 

ire, 
S ! 

iercc 

nt. 

6 

=  0          ( 

i. 

t 1 

1* 2 

■ 

. 

different  expression  from  that  used  by  Hilgard  and  by  Briggs  and 

Shantz. 

4.  The  relatively  high  wilting  coefficients  of  the  loamy  soils  having 
the  largest  humus  contents  are  believed  to  result  from  experimental 
errors,  largelv  unavoidable,  and  due  to  the  lack  of  capillar)   con- 
ductivity in  soils  which  are  particularly  loose.     This  Lack  permit 
seedling  to  succumb  in  one  region  of  the  soil,  while  there  maj  be 
considerable  free  moisture  elsewhere.     The  two  gravelly  soils 
show   similarly  high   wilting  coefficients   also   have    high    mois 
equivalents,   and  it  is  thought  from  this  that   they  were  prob 
richer  than  usual  in  permeable  feldspar,  which  could  not  bold  mucn 
water  but  would  probably  hold  it  very  firmly. 


90 


BULLETIN    1059,   U.    S.    DEPARTMENT   OF   AGRICULTURE. 


5.  It  is  to  be  noted  that  the  percentage  of  variation  of  individual 
cases  is  slightly  less  when  the  final  (diagram  5)  rather  than  the  mean 
wilting  coefficient  of  each  soil  is  taken.  On  the  other  hand,  there  is 
considerably  more  spread  between  the  two  groups  on  this  basis.  It  is 
believed  that  the  slightly  poorer  showing  made  by  using  the  mean 
wiltino-  coefficient  is  due  to  the  fact  that  losses  caused  primarily  by 
fungi  were  entirely  eliminated  from  the  calculations.  With  care 
in  this  respect,  the  mean  value  for  all  the  seedlings  is  undoubtedly 
the  more  dependable  and  also  more  expressive.  It  should  be  noted 
in  this  connection  that  in  a  group  of  100  seedlings  the  weakest  usually 


DIAGRAM    5 
RELATION     OF 

FINAL  WILTING  COEFFICIENT  TO  CAPILLARY  MOISTURE 

SAME    6ASIS  AS   DIAGRAM  4 

• 

»f 

t 

II 

(J 

10  «j 

9|  * 

'H 

Z 
u 

u. 

7 1  i 

o 

■ 

O 

u 

6 

. 

o 

z 

5f 

o 

OX* 

Li 

ir5 

^/l 

) 

2 

i 

)n ' 

&r 

) 

| 

^     a 

qB     o 

£^_4 

s 

i 

t           1 

6         2 

c 

0          ? 

!\PIL 

.AR" 

8          5 

'    MC 

J           3 

'ISTl 

6         4 

JRE, 

0      \ 

sere 
4       4 

ent 

a       5 

2         5 

6        6 

0        6 

4         6 

8          7 

Z          7 

6       6 

0        8 

4 

give  a  wilting  coefficient  twice  as  high  as  that  indicated  by  the  final 
wilting,  and  not  infrequently  three  times  as  high. 

6.  The  comparison  of  wilting  coefficients  with  moisture  equiva- 
lents shows  a  wide  gap  between  the  two  groups.  The  value  of  the 
moisture  equivalent  data  will  be  discussed  later. 

7.  While  these  results,  all  obtained  at  practically  the  same  time 
and  in  soils  which  showed  no  great  chemical  activity,  indicate  a  use- 
ful parallelism  between  wilting  coefficient  and  capillary  moisture,  it 
should  be  pointed  out  that  the  wilting  coefficient  may  occasionally 
go  out  of  bounds  as  the  result  of  acidity  or  alkalinity,  so  that  any  of 
the  physical  tests  on  soils,  taken  alone,  are  quite  worthless.  It  should 
not  be  surprising  to  obtain  wilting  coefficients  twice  as  great,  relative 
to  capillarity,  as  those  indicated  above,  especially  with  the  pines. 


RESEARCH  METHODS  IN  STUDY  OF  FOREST  E N  VI R< , X  M 1 .  \)\ 

It  is  desired  to  present  another  set  of  data  obtained  by  Bates    LO 
to  illustrate  the  need  of  establishing  the  wilting  eoefficienl 
particular  species  in  which  one  may  be  interested  and.  therefor* 
establishing  a  specific  relationship  between  the  wilting  coefficient* 
the  capillary  moistures  of  the  same  soils.     This  presentation    also 
assists  in  showing,  what  has  already  been  mentioned,  thai  a  measu 
of  the  capillarity  or  other  moisture  relation  of  the  soil  has  an  indirect 
value  in  permitting  comparisons  of  the  species  under  a   variety  of 
conditions. 

The  tests  as  presented  in  Table  3  were  performed  on  five  distinct 
kinds  of  soil,  varying  as  to  origin  (hence,  chemically)  and  also  con- 
siderably as  to  composition  and  water-holding  capacity.  With  the 
exception  of  the  prairie  soil,  which  contained  only  1  per  cent  of  coai 
sand  and  no  gravel  at  all,  these  soils  were  prepared  by  pas-mir  through 
a  sieve  with  quarter-inch  meshes. 

The  wilting  coefficient  determinations,  moreover,  were  made  with- 
out the  use  of  paraffin.  As  the  test  was  designed  particularly  to  com- 
pare, the  four  species  which  were  grown  in  each  soil,  and  it  had  he- 
come  apparent  that  the  rooting  habit  of  each  had  a  good  deal  of  heat- 
ing on  the  stage  in  soil  drying  at  which  it  succumbed,  the  effort  was 
made  to  keep  the  upper  layer  of  the  soil  well  supplied  with  moisture 
by  daily  watering.  As  a  result,  the  common  drying  of  the  stem  just 
at  the  ground  line  was  not  appreciably  in  evidence  and,  indeed,  so 
general  was  the  drying  that  the  determination  of  the  end  point  was 
exceedingly  difficult.  It  was  based  almost  wholly  on  the  flaccidity  «•( 
the  leaves.  Whether  because  of  this  protection  afforded  the  stem-  by 
surface  watering,  or  because  of  the  comparative  shade  in  which  the 
end  points  wrere  approached,  it  is  noteworthy  that  the  ratios  of  wiltii 
coefficients  to  capillarities  are  much  lower,  except  for  the  heaviest 
clay,  than  in  the  results  obtained  under  different  condition-  and 
already  described. 

Another  noteworthy  feature  of  this  test  is  that  the  seedlings  were 
produced  in  each  soil  with  the  moisture  brought  daily  to  the  moisture 
equivalent,  so  that  the  availability  was,  as  nearly  as  could  then  be 
calculated,  the  same  in  all  cases.  When  drying  began,  each  -oil 
was  brought  by  easy  stages  to  two-thirds  of  the  moisture  equivalent, 
and  finally  to  one-third.  The  seedlings  attained  an  age  about 
months  before  the  test  was  completed. 


92  BULLETIN   1059,   U.    S.    DEPARTMENT   OF   AGBICUI/TUBE. 

Table  3. —  Wilting  coefficients  of  four  species  in  five  types  of  soil. 


Sample  No.  and  description. 


590.  Sifted  granite  gravel  with  loam . 

604.  Composite  limestone 

602.  Composite  sandstone 

601.  Prairie  soil  from  shale 

603.  Composite  lava 


Moisture 
Capillary    equiva- 
moisture.       lent 
100-G. 


Averages. 


Per  cent. 
26.58 
31.85 
35.31 
37.77 
43.16 


Per  cent. 
10.55 
22.00 
21.77 
28.79 
27.80 


Mean  wilting  coefficient. 


Yellow 
pine. 


Lodge-      Douglas 
pole.  fir. 


Per  cent. 
2.50 
3.82 
5.01 
8.65 
5.56 


l'tr  cent. 
_'.  79 
4.74 
6.30 
9.65 
7.  18 


Per  c>  nt. 
2.60 
4.06 
5.08 

5.  1 I 


Engel- 
mann 
spruce. 


5.11 


6.19 


5. 1 1 


Per  cent. 
2.72 
1.03 
L87 

8.  7»V 


5.  1 1 


Sample  No.  and  description. 


590.  Sifted  granite  gravel  with 

loam 

604.  Composite  limestone 

602.  Composite  sandstone 

601.  Prairie  soil  from  shale 

603.  Composite  lava 


Yellow 
pine. 


Lodge- 
pole. 


fir. 


Ratio  wilting  coefficient  to  capillary  moisture.  Fine  material. 

I  i 

Doughs     Bpruce          A]1  snt           (.lay 

Percent.    Percent. 

0.1 1  hi  L3.5                 5.4 

.131  16.8             11.4 

.150  41.8             11. 0 

.236  53.3              17.2 

.138  51.9             11.1 


0.094 
.120 
.142 
.229 
.129 


0. 105 
.149 
.  178 

.174 


0.098 
.128 
.144 
.221 
.128 


ii.  in.' 
.127 
.138 
.  232 

.  1 23 


Averages 1428  .1721 

Mean  variations  of  single 
determinations 034ii  .  0364 

Percentage  of  mean  varia- 
tions       24.2  21.1 


.  mi 
.  0324 
22.  1 


.lilt 

.  1M 

.  03 1  - 

.0389 

"'" 

.8 

Sample  No.  and  description. 


590.  Sifted  granite  gravel  with  loam 

604.  Composite  limestone 

602.  Composite  sandstone 

601.  Prairie  soil  from  shale 

603.  Composite  lava 

Averages 

Mean  variation  of  single  determinations 
Percentage  of  mean  variations 


Ratio  of  wilting  coefficient  to  moisture 
equivalent. 


Yi-llow 
pine. 


0.237 

.  171 

2 
.301 
.200 


I.n  L'r- 

pole.  lir. 


.215 


0.247 
.185 

.196 


Spruce. 


.  l  S3 

.305 
.191 


11. -J 


.2748 

.0302 

ll.ii 


11.  J 


.  2322 

17.ii 


The  wilting  coefficient  tests  given  in  Table  3  bring  out  the  follow- 
ing facts : 

1.  The  line  showing  the  percentage  of  the  mean  variations  indicates 
that  the  four  species  taken  together  and  comprising  20  cases  have  a 
larger  variation  from  an  established  mean  ratio  than  any  of  the 
individual  species.  Lodgepole  pine  shows  the  highest  relative  wilt- 
ing coefficient,  and,  since  the  other  three  species  gave  almosl  Identical 
results,  it  follows  that  a  ratio  established  by  the  promiscuous  use  of 
species  would  be  most  largely  in  error  when  applied  to  calculations 
for  lodgepole. 

2.  The  relatively  high  wilting  coefficient  for  Lodgepole  pine  has 
been  thoroughly  established  by  numbers  of  other  tests,  which,  how- 


RESEARCH  METHODS  IN  STUDY  OF  FOREST  ENVIRONMENT. 

ever,  are  not  given  here  because  the  results  do  not  coordinate  closely 
with  this  set. 

3.  It  is  noteworthy  that  the  coefficients  for  all  species  in  the  in 
stone  soil  are  relatively  low,  while  the  really  high  vain.-  are  givi 
by  the  heaviest  clay.     The  latter  fact,  like  the  result  in  a  3trongly 
humous  soil,  is  believed  to  be  due  to  nonconductivity  of  the  cla 

4.  In  this  case  the  correlation  between  wilting  coefficients  and 
moisture  equivalent  is  a  great  deal  better  than  the  correlation  with 
capillary  moisture.  In  view  of  what  has  been  said  regarding  tl 
level  of  moisture  maintained  in  each  pan,  it  seems  pointed  to  suggesl 
that  the  wilting  coefficient  may  depend  in  some  measure  on  the  deg] 
of  moisture  to  which  the  seedlings  have  become  accustomed.  It  is 
only  logical  to  suppose  that,  if  abundant  moisture  tends  to  stimulate 
growth,  the  seedling  may,  when  drought  occurs,  be  relatively  defi- 
cient in  the  carbohydrates  which  assist  in  osmosis. 

The  moisture  equivalent  is  a  term  devised  by  Briggs  and  McLane 
(113)  to  define  the  amount  of  water  held  by  a  soil  against  a  definite 
external  force.  In  the  original  experiments  of  these  authors  the  for 
employed  was  a  centrifugal  force  exerting  a  pull  3,000  times  as  great 
as  the  force  of  gravity.  The  small  samples  of  soil  were  placed  in 
finely  perforated  cans,  which  in  turn  were  placed  against  the  inside 
wall  of  a  heavy  cylinder.  The  latter  was  caused  to  rotate  rapidly 
by  direct  connection  with  a  motor. 

In  this  early  work  the  writers  seem  to  have  made  no  attempt  to 
correlate  the  moisture  equivalents  with  wilting  coefficients.  There 
was,  however,  a  fairly  successful  formula  devised  by  which  the 
holding  power  of  the  soil  was  related  to  the  constitution  thereof,  as 
shown  by  mechanical  analyses.    This,  it  is  believed,  has  been  found 

of  little  use. 

It  remained  for  Briggs  and  Shantz  (114)  to  carry  on  the  wilting 
tests  which  showed  the  real  value  of  the  moisture  equivalent  deter- 
minations. In  these  later  tests  the  centrifugal  machine  was  con- 
siderably improved  and  its  speed  automatically  controlled,  while 
being  cut  down  to  give  a  pull  of  1,000-gravity,  since  it  was  found 
that  the  higher  tension  extracted  relatively  little  additional  water. 
As  the  result  of  some  hundreds  of  wilting  tests  and  comparisons  with 
the  moisture  equivalents  of  the  same  soils,  it  was  found  thai  from 
light  sands  to  the  heavier  clays  a  linear  relation  exists  between  tn< 
two  measures,  which  is  expressed  by  the  formula : 

moisture  equivalent 
Wilting  coefficient  =X84TT±  0.007^ 


94  BULLETIN   1059,   U.    S.    DEPARTMENT    OF    AGMCULTUBB. 

Or,  in  other  words,  there  is  a  probable  error  of  less  than  1  per  cent 
in  this  average  relationship.  Single  determinations,  however,  show 
a  probable  variation  of  2.9  per  cent  of  the  wilting  coefficient,  as 
measured  by  this  direct  means. 

This  work,  though  extremely  thorough,  was  confined  wholly  to 
soils  encoun ted  in  agricultural  regions,  and  while  these  varied  be- 
tween 1.6  to  57  per  cent  moisture  equivalent,  they  were  undoubtedly 
more  homogenous  than  forest  soils  in  general,  and  lacked  the  compli- 
cating features  of  both  rocks  and  large  quantities  of  organic  matter. 
It  is  not  desired  to  suggest  that,  if  this  method  were  readily  applicable 
to  forest  soils,  and  if  experimental  error  both  in  wilting  coefficient 
and  moisture  equivalent  determination  could  be  largely  eliminated, 
the  general  relationship  would  be  found  different  in  the  case  of 
forest  soils.  Unfortunately,  no  one  has  made  sufficient  use  of  the 
moisture  equivalent,  in  connection  with  wilting  tests  on  forest  species 
and  forest  soils,  to  determine  whether  the  formula  of  Briggs  and 
Shantz  holds  good.  It  is  hardly  to  be  doubted,  however,  that  a  for- 
mula must  be  worked  out  for  each  species,  or  the  species  of  each 
general  climatic  region.  Also,  there  is  little  doubt  that  occasional 
soils  will  be  found  in  which,  owing  to  exceptional  alkalinity  or 
acidity,  the  wilting  coefficient  is  extremely  high,  and  hence  the 
formula  breaks  down. 

In  connection  with  the  capillary  moisture  determinations  by 
Bates  (105),  data  on  corresponding  moisture  equivalents  have  also 
been  given  in  Tables  2  and  3.  These,  as  pointed  out,  were  deter- 
mined on  samples  which  had  just  passed  through  the  capillarity 
tests.  The  4  by  b\  inch  soil  cans  were  placed  in  a  machine  of  such 
speed  and  radius  as  to  develop  a  centrifugal  force  of  100-gravity, 
the  radius  being  computed  to  the  center  of  the  5-inch  column  of 
soil.  Ordinarily,  30  minutes  of  revolution  suffices  to  extract  the 
free  water  susceptible  to  this  force,  but  with  a  heavy  clay  an  hour  may 
be  required.11 

11  In  order  to  show  the  importance  of  the  time  element,  where  such  large  masses  of  soil  are  beinu  treated, 
and  also  to  illustrate  the  very  great  difference  between  the  water-holding  powers  <>f  sand  and  day,  two 
samples  were  weighed  repeatedly  after  short  periods  on  the  centrifugal  machine.  The  one  sample  con- 
sisted of  very  fine,  thoroughly  washed  sand  from  granitic  soils,  the  other  entirely  of  silt  and  clay  from  innu- 
merable sources,  the  clay  probably  not  constituting  over  one-fourth  of  the  whole  mass.  Both  samples 
had  previously  been  compacted  by  centrifuging,  so  that  the  rapid  loss  of  moisture  in  the  Oral  period  can  not 
be  ascribed  to  loose  structure.  The  test  was  somewhat  complicated  by  a  freezing  atmosphere  which,  in 
fact,  necessitated  cessation  before  an  end  point  for  either  soil  was  plainly  reached.  From  a  mass  of  soil  of 
about  1,070  grams  in  either  case,  the  sand  gave  up  in  80  minutes  276.3  grams  of  water,  of  which  230.7  grama 
(84  per  cent)  was  released  in  the  first  2|  minutes  of  centrifuging.  The  corresponding  figures  for  the  silt 
and  clay  were  76.2  grams,  and  12.7  grams  or  17  per  cent.  In  the  last  20  minutes  of  the  so-minnte  period 
the  loss  for  the  sand  was  3.1  grams  and  for  the  clay  12.8  grams. 


KESEARCH  METHODS  IN  STUDY  OF  FOREST  ENVIRONMENT. 


95 


The  results  of  tests  so  made  confirm  the  idea  which  was  given 
by  Table  2,  namely,  that  the  relation  of  the  moisture  equivalent,  as 
determined  by  a  force  of  100-gravity,  to  the  wilting  coefficient,  may 
depend  a  great  deal  on  the  type  of  soil.  To  make  the  reason  for 
this  clearer,  the  data  of  Table  2  have  been  further  grouped  in  Table 


4,  while  diagram  6  assists  in  visualizing  the  relations.  Data  for 
other  types  of  soil  have  also  been  introduced.  In  the  case  of  the 
sands  and  the  prairie  clay,  the  conditions  under  which  the  wilting 
coefficients  were  determined  were  perhaps  conducive  to  slightly 
lower  values  than  in  the  other  groups.  This,  however,  will  affect 
the  comparisons  of  wilting  coefficients  with  capillary  moisture  and 
moisture  equivalents,  about  equally. 


96 


BULLETIN    K'-.it.   U.   S.   DEPARTMENT  OF   AGRICULTURE. 


'abi  e   1       Moisture  equivalents  in  several  types  of  soil  in  relation  to  capillary  moisture 

and  wilting  coefficients. 


rip t  ion  of  group. 


Mean  Mean      Ar„an 

capil-  mois-  :x™      Mean 

'ary  ture     ™*  I    ratio 

"lols-  e?mJa-   cient.    M'E-/C 

ture.  lent. 


■I    \!  i    Nebra 

humus  ii' 'i  ovei  3      P.  ct. 

21.82 

!    Nebra 

■  to 

-43. 78 

humus 
per 

maximum) 13.13 

ruo 

r  oenl  humus 20.47 

pruce);  i 

r  cenl  humus 33.09 

ruo 

ni  humus no. 13 

7u  per  cent  silt 
and  clay,  very  little  humus.     37.77 


'UpS 

Mean     variation     be- 
roups 


P.  ct.      P.  ct. 
5.49         1-73 


19.24 

4.92 
12.62 
20.76 
43.09 
28.79 


6.99 

2.58 
3.36 
6.36 
14.86 
8.90 


0.253 

.407 

.377 
.621 
.632 
-692 
.762 


.533 
1624 


Mean 
varia- 
tion. 


0.055 

.090 

.040 
.042 
.054 
.077 


SB  S 


0.320 

.357 

.530 
.266 
.307 
.34S 
.310 


0.035 

.028 

.038 
.006 
.030 
.066 


.348 
.0543 


.034 


Mean 
Mian      varia- 
ratio        tion 
W.C.  C.  within 

group. 


0.080 

.152 

.200 
.165 
.193 
.242 
.236 


0.016 

.023 

.030 
.014 
.010 
.051 


.181 
,0419 


.021 


1 .  There  are  three  outstanding  facts  in  connection  with  these  data, 
clearly  shown  by  the  diagram.  The  first  of  these  is  that  the  two 
groups  of  sands  show  an  extremely  large  proportion  of  the  capillary 
water  removable  by  the  force  of  100-gravity,  and  correspondingly 
low  wilting  coefficients.  This  speaks  for  the  light  hold  which  the 
sands  have  on  their  moisture,  when  even  approaching  saturation. 

2.  The  second  conspicuous  fact  is  that,  with  the  exception  of  the 
granitic  gravels,  the  wilting  coefficients  and  moisture  equivalents 
rise  and  fall  somewhat  proportionately.  The  gravels  have  the 
smallesl  capacity  for  capillary  water,  a  very  weak  hold  on  a  large 
pari  of  it,  and  a  strong  hold  on  the  remainder.  This  is  partly  caused 
by  a  small  quantity  of  clay  derived  from  the  feldspar,  but  more 
largely  to  the  fact  that  the  feldspar  is  itself  somewhat  permeable. 
Coarse  cleaned  gravel  of  this  type  has  been  shown  to  have  a  capil- 
larity <»f  only  2.90  per  cent,  but  a  moisture  equivalent  of  1.70  per 

mi  .  It  seems  likely  that  practically  all  of  the  latter  would  be  non- 
available. 

3.  Another  important  point  to  be  noted  is  the  very  small  amount 
of  water  removable  from  the  prairie  clay  by  the  moderate  centrifu- 
gal Force,  and  the  correspondingly  high  wilting  coefficient, 

4  Finally,  although  the  influence  of  humus  is  somewhat  obscured 
by  the  fad  that  increasing  amounts  of  it  in  one  general  soil  type  are 
usually  accompanied  by  increasing  amounts  of  silt  and  clay,  it  seems 
fairly  certain  that  the  humus  does  not  yield  up  its  moisture  any  too 
readily  and  that  it  may  tend  to  make  the  wilting  coefficient  relatively 
high  by  preventing  capillary  movement  to  the  roots.     It  must  also 


RESEARCH  METHODS  IN  STUDY  OF  FOREST  ENVIRONMENT.         97 

be  remembered  that,  while  the  table  indicates  three  times  as  much 
water-holding  capacity  in  the  granitic  loam  of  high  humus  content, 
the  actual  increase  over  the  same  type  of  soil  with  little  humus  is  only 
about  65  per  cent  on  a  volume  basis. 

The  sum  of  these  various  effects  of  different  soil  properties  on  the 
moisture-holding  properties  is  shown  in  the  final  line  of  Table  4, 
where  it  is  clearly  indicated  that  there  is  a  closer  parallelism  between 
wilting  coefficients  and  moisture  equivalents  than  between  wilting 
coefficients  and  capillarity.  Eliminating  the  granitic  gravels,  the 
average  variation  of  the  group  ratios  is  only  7.5  per  cent  from  a 
mean  value  W.  C./M.  E.  of  0.318.  The  explanations  given,  more- 
over, all  tend  to  confirm  the  belief  that  the  moisture  equivalent 
obtained  with  a  much  greater  centrifugal  force  would  give  a  still 
closer  index  to  the  wilting  coefficient  of  any  of  these  types  of  soil. 

The  hygroscopic  coefficient  is  an  expression  of  the  amount  of  water 
held  by  a  soil  after  a  limited  exposure  to  saturated  water  vapor  under 
certain  conditions.  As  in  the  case  of  the  capillary  moisture  measure, 
it  appears  that  Hilgard  was  the  first  to  make  practical  use  of  the 
absorption  powers  of  soils,  to  compare  them  generally  as  to  phsyical 
properties,  and  to  obtain  an  approximate  measure  of  their  wilting 
coefficients.  More  recently  Alway  (102,  103)  has  done  a  large  amount 
of  work  on  this  subject,  using  Hilgard's  methods  very  largely,  but 
also  investigating  many  possible  sources  of  error  in  the  routine 
treatment  of  samples. 

It  is  a  very  well-known  fact  that  a  soil  is  never  entirely  devoid  of 
moisture  if  dried  in  the  air  for  an  indefinite  period.  On  the  contrary, 
if  atmospheric  conditions  did  not  fluctuate  so  rapidly  there  would 
be  at  all  times  an  amount  of  moisture  in  the  soil  somewhat  propor- 
tionate to  the  amount  of  vapor  in  the  atmosphere.  The  amount 
so  held  is  a  measure  of  the  soil's  hygroscopicity,  but  not  a  useful 
measure  because  of  the  changing  conditions  of  the  atmosphere. 

Similarly  a  soil  undoubtedly  still  possesses  some  hygroscopic  mois- 
ture when  dried  in  an  oven  at,  say,  100°  or  110°  C.  The  only  way  in 
which  the  soil  can  eventually  be  robbed  of  all  its  moisture  is  by 
drying  in  a  vacuum,  by  means  of  which  the  constant  withdrawal  of 
the  atmospheric  vapor  is  assured.  For  practical  purposes,  how- 
ever, drying  in  an  ordinary  atmosphere  at  110°  C.  gives  a  good  basis 
for  moisture  calculations,  since  at  that  temperature  the  vapor  in  the 
atmosphere  will  be  very  much  rarefied  in  comparison  with  its  satura- 
tion capacity.  This  point  is  mentioned  because  it  is  not  infrequently 
noted,  in  drying  large  samples,  that  they  may  gain  moisture  in  the 
hot-air  oven  if  there  is  a  decided  increase  in  atmospheric  moisture. 
To  avoid  appreciable  errors  it  has  been  found  necessary  to  avoid 
final  weighings  of  oven-dried  samples  on  excessively  moist  days. 
10163— 22— Bull.  1059 7 


98  BULLETIN    1059,  IT.   S.   DEPARTMENT  OF   AGRICULTURE. 

The  discovery  that  within  certain  limits  the  moisture  of  the  soil 
follows  the  laws  of  osmosis,  or  more  precisely  speaking  the  laws  of 
dilute  solutions  with  respect  to  its  freezing  point,  has  naturally  led 
to  the  idea  that  the  soil  solution  might  also  be  considered  as  having 

lefinite  vapor  pressure  at  a  definite  osmotic  concentration.     If  this 

re  true,  then  a  soil  placed  in  a  moist  atmosphere  should  give  off 
or  absorb  vapor,  according  to  whether  its  original  solution  repre- 
3ented  a  lower  or  higher  osmotic  pressure  than  that  represented  by 
the  atmosphere  of  vapor  in  which  it  was  placed.  Furthermore,  if 
this  vapor  pressure  manifested  itself  properly  and  in  accordance 
with  the  laws  of  solutions,  then,  through  vapor  transfers,  one  soil  or 
a  hundred  soils  simultaneously  might  be  brought  into  vapor-pressure 
equiUbrium,  and  thereby  into  osmotic  equilibrium,  with  a  solution 
whose  osmotic  pressure  is  readily  determined;  and  the  moisture  con- 
tents corresponding  to  such  osmotic  pressure  might  then  be  readily 
measured  for  one  or  all  of  the  soils.  This  plan  was  conceived  as  a 
possible  means  of  avoiding  some  of  the  difficulties  of  the  freezing- 
point  method  of  osmotic  determinations,  which  are  especially  bother- 
gome  in  treating  coarse  soils.  That  the  theory  is  correct  may  hardly 
be  questioned  now,  and  full  discussion  of  the  available  data  will  be 
given  later.     This  subject  has  been  mentioned  here  because  of  its 

>sible  hearing  on  the  hygroscopic  coefficient  determination-.  It  is 
lather  readily  seen  that,  if  the  laws  of  solutions  prevailed  under  all 
conditions  of  soil  moisture,  a  soil  exposed  to  completely  saturated 
water  vapor  should  go  on  absorbing  moisture  indefinitely,  because 
the  dilute  solution  of  the  soil  would  always  stand  for  some  osmotic 
pressure,  while  saturated  water  vapor  would  stand  for  none  at  all. 

Whether  this  does  not  occur  in  the  hygroscopicity  tests  because  of 
the  failure  to  create  a  completely  saturated  atmosphere,  or  because 
there  is  a  sharp  line  between  the  behavior  of  water  vapor  in  the  soil 
and  liquid  water,  is  for  the  future  to  decide.  That  it  probably  has 
no  practical  bearing  on  the  hygroscopic  coefficient  under  the  empiric 
conditions  set  for  that  test  is  perhaps  enough  in  itself.  It  will 
h.-lp  to  clarify  the  matter  if  it  is  remembered,  first,  that  Bouyoucos 
109)  has  showm  that  at  about  the  moisture  content  at  which  wilting 
occurs,  the  water  of  the  soil  ceases  to  behave  as  a  liquid  and  refuses 
to  freeze;  and  secondly,  that  Briggs  and  Shantz  (114)  have  shown 
that  the  hygroscopic  coefficient  falls  considerably  below  the  wilting 
coefficient,  the  former  being  usually  about  0.7  of  the  magnitude  of  the 
latter. 

Smce  the  determination  of  the  hygroscopic  coefficient  begins  with 
air-dry  soil,  it  does  not  deal  with  liquid  water  in  the  soil,  but  more 
probably  with  water  molecules  more  or  less  separated,  like  individual 
vapor  molecules. 


RESEARCH  METHODS  IX  STUDY  OF  FOREST  ENVIRONMENT.         99 

The  conditions  under  which  the  hygroscopic  coefficient  should  be 
determined,  as  most  recently  worked  out  by  Alway,  Kline,  and 
McDole  (103),  are  briefly  as  follows: 

1.  The  absorption  box  is  of  wood,  12  by  9  by  8  inches,  the  interior 
surfaces  being  paraffined  to  prevent  absorption  of  water  and  warp- 
ing.    Larger  boxes  were  found  to  be  more  difficult  to  keep  saturated. 

2.  In  the  bottom  of  this  box  is  placed  a  snug-fitting  galvanized-iron 
tray,  3  to  4  inches  high,  to  hold  the  water.  The  walls  of  the  box  are 
then  lined  with  blotting  paper,  the  edges  of  which  project  into  the 
vessel  of  water.  This  insures  rapid  dissemination  of  vapor  through 
the  interior. 

3.  A  wooden  table  is  held  by  metal  supports,  1  to  2  inches  above  the 
surface  of  the  water  in  the  tray.  On  this  table  are  placed  the  two 
trays  which  hold  the  soil  samples. 

4.  Metal  trays  are  accepted  as  most  satisfactory,  because  they  ab- 
sorb no  moisture  and  hence  do  not  retard  absorption  by  the  soil. 
These  trays  are  of  aluminum  or  copper,  7  inches  long,  5  inches  wide, 
and  0.75  inch  deep. 

5.  The  soil  is  carefully  sifted  over  the  bottom  of  the  tray  to  a  depth 
of  1  millimeter.  This  naturally  precludes  the  use  of  coarser  material. 
It  was  found  that  there  was  little  or  no  change  in  soils  from  careful 
grinding  which  would  barely  permit  the  coarser  particles  to  pass  a 
1-millimeter  sieve.  It  was  also  found  that  oven  drying  at  105°  to 
110°  C.  did  not  appreciably  affect  the  absorbing  capacity  of  any  of 
the  soils  tested.  In  any  case,  however,  previous  drying  should  be 
avoided  when  possible. 

6.  The  exposure  to  vapor  in  the  boxes  is  for  24  hours.  At  the  end 
of  this  period  the  soil  tray  is  removed  from  the  box  as  quickly  as 
possible  and  emptied  into  a  stoppered  weighing  bottle,  since  exposure 
to  the  air  beyond  a  few  seconds  would  cause  appreciable  loss  of  mois- 
ture. 

7.  Undoubtedly  the  most  important  consideration  in  securing 
reliable  results  is  a  suitable  room.  This  must  be,  in  most  cases,  a  cellar 
room  not  subject  to  daily  fluctuations  of  temperature  or  heating  from 
one  side,  or  even  to  localized  heating  from  bright  light.  These  pre- 
cautions are  absolutely  vital,  if  condensation  is  to  be  prevented.  As 
a  matter  of  theory,  it  is  altogether  probable  that  the  need  is  to  pre- 
vent even  momentary  complete  saturation  of  the  vapor  in  proximity 
to  the  soils,  since  this  might  give  rise  to  the  creation  of  liquid  moisture 
in  them,  and  entirely  alter  their  condition. 

8.  A  temperature  of  about  60°  F.  may  be  considered  a  standard. 
At  a  lower  temperature  there  will  be  fewer  water  molecules  reaching 
the  soil,  and,  necessarily,  a  slower  rate  of  absorption. 


100         BUIXETIN   1051),   U.   S.   DEPARTMENT  OF   AGRICULTURE. 

Under  these  circumstances  fairly  constant  results  may  be  expected 
in  i,  coefficient   determinations.     In  the    absence  of  any 

other  results  reference  is  again  made  to  the  comparisons  made  by 
Bi  Shantz  between  hygroscopic  and  wilting  coefficients.     In 

17  itli  soils  varying  from  0.9  to  16.5  per  cent  wilting  coeffi- 

ru.nl    j  |  miihI  the  ratio  of  hygroscopic  to  wilting  coefficients  to  be 

average  0.680,  with  a  probable  error,  or  variation,  in  any 
single  determination  of  about  7.1  per  cent  of  the  wilting  coefficient. 
It  is  to  be  noted  that  the  hygroscopic  is  so  much  lower  than  the 
u  ih  in-  coefficient  that  serious  error  would  result  from  considering 
them  as  interchangeable,  though  this  proposal  has  sometimes  been 

made. 

Calculation  of  the  Available  Moisture. 

&s  has  been  stated,  when  the  current  moisture  of  the  soil  has  been 
measured,  and  the  nonavailable  has  been  measured  in  the  laboratory 
by  the  direct  method  of  wilting  tests,  or  indirectly  through  the 
capillary  moisture,  moisture  equivalent,  or  hygroscopic  coefficient,  it 
is  then  only  necessary  to  subtract  the  wilting  coefficient  from  the 
whole  moisture  to  have  a  measure  of  the  amount  of  water  which, 
under  the  most  favorable  circumstances,  will  be  available  for  growth. 
For  example,  if  in  sand  and  clay,  respectively,  the  whole  moistures 
are  1<>  and  20  per  cent,  and  the  wilting  coefficients  of  these  soils 
are  respectively  2  and  15  per  cent,  then  it  is  evident  that  in  the 
-.tnd  there  is  8  per  cent  available  moisture,  and  in  the  clay  5  per 

•i! .  or  .1  =M—WO.  The  use  of  the  last  figures  is  certainly  far  more 
expressive  of  the  relative  conditions  in  the  two  soils  than  would  be 
the  u-«'  <»f  the  whole  moisture  figures,  although,  on  account  of  vary- 
ing concent  rat  ions  of  salts,  even  this  figure  for  the  available  moist  ore 
does  not  give  a  direct  means  of  comparing  the  moisture  conditions 
of  radically  different  soils. 

Of  course,  if  the  measure  of  available  moisture  is  to  be  used  most 
fully  as  an  index  to  supply,  the  percentage  should  be  transposed 
linall\   into  cubic  centimeters  per  cubic  meter,  or  any  other  measure 

\    \  ollline. 

This  is   very  readily  done  if  the  apparent   density   has  been   de- 

i  mined,  as  in  the  large  capillary  cans  described,  where  the  apparent 

density  is  obtained  by  dividing  the  dry-soil  weight,  in  grains,  by  the 

>lume   in   cubic  centimeters,  which  is  approximately    1,030   cubic 

centimeters  (usually  less  after  centrifuging). 

I  arrying  the  volume  idea  still  farther,  in  studying  any  plant   or 
oup  of  plants  it  is  obviously  desirable  to  know  how  much  soil  sur- 
face can  be  drawn  upon.     Thus  a  yellow  pine  on  a  dry  site  may 
actually  have  a  much  greater  supply  of  moisture  than  a  crowded 
spruce  on  a  moist  site.     Consideration  of  this  point  of  view  will  lead 


RESEARCH  METHODS  IX  STUDY  OF  FOREST  ENVIRONMENT.      101 

to  the  conclusion  that  soil  moisture  figures,  as  ordinarily  given  in 
percentages  of  the  dry-soil  weight,  have  almost  no  significance 
ecologically.  Both  area  and  depth  of  soil  which  contribute  to  a  given 
plant  must  be  known. 

Availability  of  the  Moisture. 

As  already  stated,  it  is  intended  to  confine  this  term  u  availability' 
to  the  simple  relation  between  the  whole  moisture  and  the  available 
moisture.  The  term  can  not  be  an  exact  expression  of  the  rate  at 
which  the  paint  will  be  able  to  obtain  water,  since  such  rate  depends 
on  conditions  within  the  plant  as  well  as  those  without;  or,  in  brief, 
on  the  need  of  the  plant  for  water.  It  should,  however,  have  greater 
value  than  a  bare  measure  of  whole  moisture,  or  even  of  available 
moisture  in  percentage  of  dry  soil  weight  as  an  expression  of  a 
condition  of  the  soil.  Its  value  is  predicated  on  the  assumption 
that,  at  the  wilting  point,  a  given  plant  is  probably  exerting  a  fairly 
definite  osmotic  pressure  in  its  effort  to  obtain  water,  and  that  at 
this  time  the  osmotic  pressure  of  the  soil  water  is  also  definite  and  the 
same  as  that  in  the  plant.  This  is  evidently  not  the  case  if  the 
wilting  coefficient  is  as  low  as  the  point  at  which  both  the  water  and 
solutes  become  absorbed  by  the  soil  colloids,  for  at  this  point  the 
osmotic  pressure  becomes  infinitely  large.  For  this  reason  the  pro- 
posed measure  of  availability  may  have  only  limited  usefulness,  but 
should  at  least  serve  as  a  stepping  stone  to  the  next  and  more  definite 
proposal. 

If,  for  example,  it  is  assumed  that  when  a  plant  wilts  it  is  exert- 
ing an  osmotic  pressure,  P,  of  100  atmospheres,  then  supposedly  at 
the  same  time  (that  is,  in  the  condition  expressed  by  the  wilting 
coefficient)  the  soil  is  exerting  an  opposing  force,  P',  also  repre- 
sented by  100  atmospheres.  If,  then,  an  amount  of  water  equal  to 
the  wilting  coefficient  is  added  to  the  soil,  the  soil  solution,  roughly 
speaking,  has  been  diluted  to  one-half  its  previous  strength,  and 
there  is  a  differential  in  favor  of  the  plant  of  50  atmospheres.  Since 
the  starting  point  was  100  atmospheres,  this  situation,  or  the  avail- 
ability for  this  particular  plant  and  soil,  may  be  expressed  as 
50/100  or  0.50.  Similarly,  when  the  moisture  content  is  three  times 
the  wilting  coefficient,  P'=33  atmospheres,  the  differential  is  67 
atmospheres,  and  availability  is  0.67.  It  is  seen  that  this  is  readily 
expressed  by 

M-WC 


Av.= 


M 


giving  availability  numerical  values  somewhat  proportionate  to  the 
osmotic  pressures  in  favor  of  the  plant. 


102  BULLETIN    1059,    U.   S.    DEPARTMENT    OF   AGRICULTURE. 

THE    COEFFICIENT    OF   AVAILABILITY. 

V<  already  suggested,  the  ability  of  the  plant  to  supply  itself 
with  water  would  seem  to  be  measurable  in  terms  of  the  differential 
between  the  osmotic  pressure  within  the  plant  and  the  antiosmotic 
pressure  exhibited  by  the  soil  moisture,  with  an  allowance  for  the 
distance  through  which  this  force  must  operate.  This  same  basis 
was  used  in  the  preceding  section  as  a  rough  means  of  showing 
changes  in  the  soil  condition,  but  without  any  allowance  for  changes 
in  the  absorbing  power  of  the  plant  which  occur  with  its  loss  or 
Min  of  water,  or  without  considering  the  factor  of  height  and  dis- 
tance  as  it  may  affect  tall  trees. 

In  attempting  thus  to  express  the  availability  of  water  to  the 
plant,  in  precise  terms  or  osmotic  pressures,  currently  for  any  con- 
dition that  may  be  encountered  in  the  soil  or  plant,  it  is  necessary 
to  determine  the  osmotic  pressure  of  the  soil  or  plant  quickly  and 

accurately. 

The  osmotic  pressure  of  an  aqueous  solution  is  determined  by  the 
increase  in  its  boiling  point  over  that  of  pure  water:  by  the  depression 
of  n-  freezing  point;  by  the  decrease  in  the  vapor  pressure  over  the 
-  >lution;  and,  possibly,  by  the  increase  in  the  latent  heat  of  vapori- 
zation. It  is  only  recently  that  investigations  of  the  last  have  been 
made,  so  that  there  is  no  known  formula  which  would  make  this 
process  available. 

Within  the  limits  of  so-called  dilute  solutions  a  rise  of  1°  C.  in  the 
boiling  point  represents  an  osmotic  pressure  of  about  ~u  atmos- 
phere-; a  depression  of  11°  C.  in  the  freezing  point  indicates  P  = 
L2.05  atmospheres,  and  a  depression  of  1  per  cent  in  the  saturated 
vapor  pressure  over  the  solution,  the  temperature  being  the  same, 
indicates  about  12  atmospheres  pressure.  These  approximate  figures 
permit  ns  to  judge  of  the  practical  utility  and  accuracy  of  different 
method-. 

It  may  also  be  useful  at  this  point  to  refer  to  the  fact  that  in  pure 
solutions,  such  as  may  be  used  in  the  vapor-transfer  method  or  in 
plasmolytic  tests  on  tissues,  the  osmotic  pressure  is  very  readily 
determined  by  the  concentration  of  the  solution,  in  terms  of  the 
molecular  weight  of  the  solute,  provided  the  solute  is  chemically  pure 
and  anhydrous.  According  to  Nernst  (134)  the  "molecular  lowering 
of  the  freezing  point"  for  water  is  18.4°  C.,12  or  1.84°  C.  when  1 
gram  molecule  of  the  substance  is  dissolved  in  a  Liter  of  water.  A  1- 
molecule  solution,  therefore,  stands  for  22.12  atmospheres  osmotic 
pressure. 

From  these  data  it  would  seem  that  the  boiling-point  method 
would  insure  the  greatest  precision  in  osmotic  pressure  determina- 

More  recent  investigations  reported  by  Jones  U0  show  that  the  molecular  lowering  may  be  iwice  this 

SSJ  tmt^Cfe°fstltswhicllaredissociatedl)ywa^^totwoion!       !  |  letenninat* 

should  quickly  decide  this,  in  case  of  doubt. 


RESEARCH  METHODS  IN  STUDY  OF  FOREST  ENVIRONMENT.      103 

tions,  were  it  not  for  the  effect  on  the  boiling  point  of  the  character 
of  the  vessel  itself,  of  gases  in  the  liquid,  and  of  solid  particles  which 
form  nuclei  for  steam  bubbles.  It  is  also  self-evident  that  the  boiling- 
point  method  is  not  applicable  to  soils,  and  hardly  more  applicable 
to  plants  unless  the  sap  has  been  separated  from  the  pulp,  which, 
under  certain  circumstances,  as  in  the  treatment  of  conifers,  may  be 
impossible  of  attainment. 

The  freezing-point  method  comes  next  in  order,  and  has  been  con- 
siderably used;  though,  at  this  stage,  it  is  well  to  mention  that  the 
foliage  of  coniferous  trees  frequently  becomes  so  dry  that  a  definite 
freezing  point  can  not  be  determined,  probably  because  of  the  lack 
of  conductivity  in  the  mass,  which  is  such  that  each  particle  of  the 
pulp  may  freeze  without  affecting  the  rest  of  the  mass  quickly. 

The  vapor-pressure  method  does  not  look  so  promising,  because 
of  the  technical  difficulties  in  the  way  of  any  precise  determination  of 
vapor  pressure.  However,  the  complicated  apparatus  necessary  for 
the  direct  determination  of  a  vapor  pressure  may  be  done  away 
with  if  instead  the  determination  of  vapor  pressure  is  made  in  a 
vessel  by  means  of  a  solution  which  is  in  equilibrium  with  that  vapor. 
This  method  especially  commends  itself  in  the  treatment  of  soils 
because  of  the  possibility  of  preparing  them  and  retaining  them  dur- 
ing treatment  in  a  state  of  compactness  and  granulation  approaching 
the  natural.  It  does  not  seem  so  applicable  to  plant  tissues  because 
of  the  danger  of  fermentation  and  enzymic  action  during  the  treat- 
ment. 

The  determination  of  osmotic  pressures  in  plant  cells  by  plasmolysis, 
while  evidently  useful  for  the  examination  of  restricted  areas,  such 
as  the  epidermis,  and  possibly  useful  for  any  tissues  which  are  ex- 
ceedingly dry,  does  not  recommend  itself  for  general  purposes  be- 
cause of  the  large  amount  of  manipulation  necessary  and  the  experi- 
mentation required  to  find  the  balancing  solution.  This  method,  of 
course,  necessitates  the  observation  of  individual  cells  under  the 
microscope,  when  placed  in  media  of  various  osmotic  concentrations. 

Osmotic  Pressure  of  Plant  Tissues. 

Dixon  and  Atkins  (119),  in  1913,  were  apparently  the  first  to  use 
the  then  developing  theoretical  knowledge  of  the  behavior  of  solu- 
tions as  a  means  toward  looking  into  the  internal  conditions  of  plants. 
In  the  citation  given  they  deal  at  length  with  the  method  of  extract- 
ing sap  from  plant  tissues  for  the  purpose  of  freezing-point  determi- 
nations. 

Hibbard  and  Harrington  (126),  in  1916,  and  Harris,  Lawrence, 
and  Gortner  (123),  in  the  same  year,  followed  this  work  with  further 


104  BULLETIN   1050,   U.    S.    DEPARTMENT  OF   AGRICULTURE. 

studies  of  the  method  of  determining  the  freezing  point  of  cell  saps, 
and  also  applied  this  knowledge  to  the  study  of  plants  under  various 
habital  conditions.  The  latter  were  probably  the  first  to  point  out 
that  the  osmotic  pressure  of  the  cell  sap  varied  rather  directly  with 
the  dryness  of  the  habitat.  They  also  showed  that  trees  and  shrubs 
possess  higher  pressures  than  the  lower  and  shorter  lived  forms  of 
vegetation,  which  furnishes  the  basis  for  considering  height  as  a 
factor  affecting  the  asmotic  pressure  in  the  leaves. 

McCool  and  Millar  (131),  1917,  experimented  with  impressed  plant 
tissues  and  obtained  practically  the  same  results  as  when  the  ex- 
tracted >aps  were  used.  This  was  a  distinct  step  forward  in  simplify- 
ing the  process,  and  therefore  no  attempt  is  made  to  describe  the 
method  of  sap  extraction.  McCool  and  Millar  found  it  only  neces- 
sary to  macerate  slightly  the  material  with  a  stiff  wire,  in  the  freezing 
tube.  These  investigators  also  brought  out  much  new  information 
on  the  changes  in  osmotic  pressure  in  the  leaves  with  atmospheric 
changes,  and  the  close  correlation  between  root  pressures  and  condi- 
tions of  the  soil  moisture,  the  former  being  little  influenced  by  atmos- 
pheric conditions. 

Bates  105),  in  1917,  seeking  an  explanation  of  the  great  difference 
in  the  transpiring  capacity,  of  different  species  of  tree  seedlings,  and 
not  being  equipped  with  freezing-point  apparatus,  obtained  the  sap 
density  of  the  aerial  portions  of  whole  seedlings  by  grinding  bhem  in 
a  food  grinder,  extracting  the  water-soluble  substances,  filtering  the 
liquid,  and  then  drying  the  water-soluble  solids  and  the  washed  pulp 
separately.  The  weight  of  these  two,  when  deducted  from  the  origi- 
nal weight  of  the  plant,  gives  the  weight  of  the  original  solvents,  and 
the  'sap  density'  is  expressed  by  the  ratio  between  solute-  and 
solvent.  These  first  results  were  found  to  have  a  close  relation  to 
the  transpiration  rates  that  had  been  observed,  and  it  was  therefore 
concluded  that  sap  density  might  very  largely  serve  as  an  automatic 
restriction  on  transpiration. 

Although  realizing  that  an  expression  of  osmotic  pressure-  would 
give  a  more  reliable  basis  for  comparing  the  species,  this  was  not 
undertaken  for  some. time,  since  it  was  desired  to  establish  first  the 
importance  of  the  sap  density  as  a  measure  of  the  condition  of  the 
plan!  and  its  response  to  various  atmospheric  condition-.  Tin-  work 
ha-  beep  pursued  to  some  extent.13  It  is  only  desired  here  to  <tate 
that,  within  the  limits  of  experimental  error,  the  osmotic  pressures 
shown  by  a  number  of  the  conifers  appear  to  be  the  same  when  the 

p  densities  by  the  above  method  are  the  same.     Considering  all  of 


"Forest  Types  of  the  Central  Rocky  Mountains,"  by  C.  G,  Bat<        Unpublished  report. 


RESEARCH  METHODS  IN  STUDY  OF  FOREST  ENVIRONMENT.      105 

the  species,  the  following  correlation  between  the  two  measures  is 
given  tentatively: 


Sap 
density. 

Osmotic 
pressure. 

Per  cent. 

•") 
10 
15 

Atmosplu  ri . 

10.0 
to.  7 
22.  4 

For  rough  approximations,  the  osmotic  pressure  in  atmospheres 
may  be  considered  equal  to  the  sap-density  percentage  for  plants  of 
this  class.  It  is  probable  that  further  data  will  bring  out  specific 
differences  worthy  of  consideration.  It  is,  therefore,  believed  that 
where  freezing-point  determinations  are  impracticable  because  of 
lack  of  apparatus,  or  of  freezing  mixtures;  or  when,  as  frequently 
happens  with  the  foliage  of  conifers,  the  material  is  so  dry  that  even 
with  gnnging  it  lacks  free  moisture,  so  that  a  distinct  end  point  can 
not  be  secured,  the  sap-density  method  may  be  of  very  great  assist- 
ance. 

Af  ter  considerable  experimentation  with  a  number  of  methods  giv- 
ing essentially  the  same  results,  the  following  simple  practice  has 
been  developed,  which  is  designed  primarily  to  eliminate  the  need  for 
evaporating  the  large  volume  of  water  used  in  extracting  the  solutes; 
it  also  greatly  reduces  the  opportunity  for  loss  of  material  during  the 
operation. 

1.  The  plant  material,  usually  consisting  of  the  more  exposed  and 
consequently  the  drier  portion  of  the  needles,  is  secured  by  carrying 
into  the  field  the  desired  number  of  wide-mouthed  liter  flasks,  a  fun- 
nel, and  a  pair  of  shears.  The  plant,  or  branch  of  a  tree,  is  held  over 
the  funnel,  and  the  leaves  are  snipped  off  in  sections  not  over  one- 
half  inch  long,  the  outer  one-half  to  two-thirds  of  all  needles  being 
taken.  When  10  to  15  grains  of  material  has  been  secured,  the  flask 
is  stoppered.  As  soon  as  a  collection  has  been  completed,  the  flasks 
are  taken  in  and  weighed  with  their  contents,  the  flask  weights  hav- 
ing previously  been  recorded.  Confusion  may  be  avoided  by  making 
all  weighing  with  the  stoppers  removed. 

2.  The  flasks  are  now  placed  in  the  drying  oven  for  a  period  of  not 
less  than  12  hours.  It  will  usually  be  found  convenient  to  have  the 
specimens  ready  for  extraction  early  in  the  morning.  At  this  stage 
the  weight  of  flask  and  dry  contents  is  secured,  and  by  the  difference 
between  this  and  the  earlier  weight,  the  original  water  content  is 
obtained  directly. 

3.  Each  flask  now  has  added  to  it  distilled  water  to  the  extent  of 
five  times  the  weight  of  the  green  plant  material  and  is  then  again 
placed  in  the  oven  for  an  hour,  the  temperature  attained  in  this  time 


10f,  BULLETIN    1059,    U.    S.   DEPARTMENT   OF   AGRICULTURE. 

being  practically  that  of  boiling  water,  which  will  greatly  facilitate 
the  diffusion  of  the  solutes.  While  this  warming  is  being  accom- 
plished, a  filter  may  be  prepared,  corresponding  to  each  flask.  The 
filter  papers  are  dried  and  weighed  before  using,  and  their  weights 
credited  to  the  samples  with  which  they  will  be  used.  At  the  end 
of  the  hour  the  liquid  contents  of  each  flask  are  poured  into  their 
appropriate  filters. 

I.  Water  is  again  added  to  each  flask,  in  the  same"  amount  as  be- 
fore,  and  the  process  repeated.  After  the  third  extraction,  with 
possibly  a  little  cold  rinsing  of  the  pulp,  filter,  etc.,  the  filter,  with 
whatever  solids  it  has  accumulated,  is  placed  in  the  flask,  and  this 
i-  ict  urned  to  the  oven  for  its  final  drying.  After  this,  making  allow- 
ance for  the  filter-paper  weight  that  has  been  added,  the  loss  of 
solute-  is  readily  computed.  It  should  also  be  realized  that  this  loss 
will  include  a  small  proportion  of  water  which  was  hygroscopicallv 
held  by  the  solutes  in  the  previous  drying.  Such  loss,  however,  will 
possibly  compensate  for  solutes  not  removed.  While  the  three  ex- 
tractions, theoretically,  should  remove  more  than  99.9  per  cent  of 
the  solids,  it  is  probable  that  they  fall  appreciably  short  of  this. 

Since  it  is  believed  that  investigations  along  this  line  will  de- 
velop increasing  importance  in  forest  ecology,  it  seem-  advisable  to 
make  available  a  table  of  osmotic  pressures  for  freezing-point  de- 
pressions to  5.999°  C,  as  worked  out  by  J.  A.  Harris  and  published 
m  the  American  Journal  of  Botany,  2:418-419,  1915.  This  is  an 
extension  of  the  work  begun  by  Harris  and  Gortner  in  1914.  The 
table  will  be  found  in  the  appendix. 

Of  almost  equal  ecological  importance  with  the  increase  in  osmotic 
pressure  and  absorbing  capacity  which  accompanies  greater  concen- 
tration of  the  cell  sap,  is,  perhaps,  the  very  great  decrease  in  the  prob- 
able rate  of  evaporation  from  the  leaves.  It  is  especially  desired  to 
<-all  attention  to  this,  since  the  earlier  announcement-  of  the  findings 
of  physical  chemistry  have  led  many  biologists  to  believe  that 
a  considerable  change  in  the  osmotic  pressure  of  the  plant  solu- 
tion could  have  little  effect  on  evaporation.  Thus  Livingston 
130),  in  1911,  argued  that  the  greatest  concentration  of  the  cell  sap 
would  only  create  a  depression  of  10  per  cent  in  the  vapor  pressure 
over  the  solution,  and  consequently  could  have  no  important  effeel 
on  the  evaporation  rate. 

As  early  as  1915  Bates  (105)  had  observed  in  the  artificial  drying 
of  pine  cones,  for  which  a  calorimetric  kiln  was  used,  a  very  greal 
increase  in  the  amount  of  heat  consumed  as  the  drying  advanced'.  In 
certain  instances  this  was  nearly  three  times  as  great,  per  unit  of 
water  evaporated,  in  the  final  stages  as  when  beginning  with  very 
green  cones.  When,  therefore,  he  found,  in  1917.  a  great  decrease 
m  the  transpiration  rate  of  those  species  of  conifers  which  showed 


RESEARCH  METHODS  IX  STUDY  OF  FOREST  ENVIRONMENT.      107 

the  highest  concentration  of  the  cell  sap,  he  was  led  to  investigate 
this  matter  further,  Finding  nowhere  any  reference  to  experiments 
on  the  latent  heat  of  vaporization  of  solutions,  and  believing  that  the 
conception  of  the  fixed  nature  of  that  quantity  for  water  was  based 
upon  the  fact  that  the  condensation  of  steam  had  always  been  em- 
ployed to  determine  it,  he  has  been  led  to  perform  a  number  of  ex- 
periments with  solutions  and  with  distilled  water. 

The  most  important  and  convincing  of  these  shows  that  at  the 
respective  boiling  points  of  water,  and  various  solutions  up  to  the 
point  of  saturation  (for  sodium  chloride),  the  latent  heat  of  vapori- 
zation, determined  directly  by  means  of  an  electric  heating  element, 
is  practically  a  constant,  though  perhaps  varying  inversely  as  the 
absolute  boiling  point.  Thus  a  saturated  salt  solution  whose  boiling 
point  is  7°  above  that  of  water  and  whose  osmotic  pressure  is  theo- 
retically about  400  atmospheres,  requires  only  4  per  cent  less  heat, 
per  unit  of  water  evaporated,  than  does  pure  water.  This,  however, 
does  not  solve  the  problem,  as  will  be  seen  from  the  fact  that  when 
placed  over  a  steam  bath  the  saturated  salt  solution  evaporates  at 
a  rate  of  less  than  5  per  cent  of  that  for  pure  water.  There  is  in  the 
problem,  therefore,  very  evidently  some  factor  besides  vapor  pres- 
sures and  latent  heats  of  vaporization  when  an  external  supply  of 
heat  is  concerned.  It  appears  to  be  a  matter  of  conductivity  and 
possibly  also  of  convection.  Further  investigation  of  the  problem 
is  urgently  needed. 

Method  of  determining  freezing  -points. — Since,  as  has  been  stated, 

the  treatment  of  the  leaves  of  forest   trees,  especially  conifers,  is 

likely  to  present  some  complications  because  of  the  extreme  dryness 

which  they  sometimes  show,  it  is  believed  the  whole-tissue  method 

of  McCool  and  Millar  (131)  is  likely  to  be  ineffective.     Hibbard  and 

Harrington  (126)  are  therefore  quoted  here  on  the  process  used  by 

them  and  involving  grinding  of  the  frozen  tissues.     From  this  basis 

any  investigator  will  certainly  be  able  to  devise  modifications  to  suit 

his  special  conditions. 

The  apparatus  used  in  our  tests  was  the  Beckmann  outfit  ordinarily  used  for  such 
work  and  described  in  books  on  physical  chemistry,  consisting  of  a  Beckmann  ther- 
mometer, freezing  tube,  outer  jacket,  and  a  battery  jar  containing  the  freezing  mixture. 
The  freezing  point  of  distilled  water  was  taken  as  zero,  and  the  lowering  of  the  freezing 
point  of  the  pulp  was  obtained  by  subtraction.  When  determining  the  freezing  point 
of  distilled  water  an  electric  stirring  device  was  used  consisting  of  battery,  metronome, 
magnet,  and  platinum  stirrer,  but  this  was  not  employed  in  determinations  made  upon 
pulps.  The  pulp  was  allowed  to  undercool  about  1°,  after  which  the  beginning  of 
solidification  was  brought  about  by  rotating  the  thermometer  backward  and  forward 
a  few  times  in  the  pulp.  When  the  undercooled  mass  of  pulp  was  thus  disturbed 
the  temperature  began  to  rise  almost  immediately  and  soon  came  to  rest,  after  which 
the  thermometer  was  tapped  several  times  and  the  final  reading  then  taken.     This 


108  BULLETIN    1059,    U.    S.   DEPARTMENT   OF   AGRICULTURE, 

reading  was  considered  as  the  freezing  point  of  the  pulp  tested.     Correction  for  under- 

oljng  has  een  applied,  since  the  undercooling  was  always  the  same.     Since, 

illy  emphasized  by  Shive14  the  external  air  temperature  exercu 

■  on  the  apparent  depressions  of  the  freezing  point  obtained  by  means 

of  the  Beckmann  apparatus,  The  freezing  of  the  pulp  or  expressed  juice  must  always 

I  out  with  approximately  the  same  temperature  of  the  surrounding  air  as  pre- 

Iled  during  the  determination  of  the  freezing  point  of  distilled  water  used  for 

>n  wiili  that  of  pulp  or  juice.     The  simplest  way  to  avoid  possible  sources  of 

n  this  connection  is  to  make  a  freezing-point  determination  on  distilled  water 

for  each  external  air  temperature  at  which  pulps  or  juices  are  tested.     Then  the 

l,,v  For  any  test  is  considered  as  the  difference  between  its  freezing  point  and 

that  obtained  on  distilled  water  with  the  same  room  temperature. 

The  property  of  the  solution  upon  which  its  maximum  possible  osmotic  pres- 
sure depends  is  approximately  measured  by  its  freezing  point  lowering,  and  this 
property  may  he  expressed  in  terms  of  pressure.  Thus,  according  to  the  formula  of 
Lewis,15  11=12. 06A—  0.021  A,2  where  His  the  maximum  osmotic  pressure,  in  atmos- 
pheres,  at  the  freezing  point  of  the  solutions,  and  A  is  the  lowering  of  the  freezing 
point  in  centigrade  degrees,  below  that  of  distilled  water.  With  the  aid  of  this  formula 
Harris  and  Gortner  16  have  prepared  a  table  of  the  values  of  II  for  the  range,  A  =0.001° 
( '.  to  A^--5.999°  C.     This  table  has  been  employed  in  our  deductions. 

At  the  beginning  of  the  work  the  material  to  be  used  was  first  ground  and  then 
frozen,  but  it  was  difficult  to  prevent  some  loss  of  sap  in  this  way.  and  difficulty  was 
also  encountered  in  getting  a  perfect  mixture  of  the  material  after  thawing,  since 
much  of  the  sap  had  left  the  cells  on  grinding  and  had  sett  lei  1  to  the  bottom  of  the 
mass.  Consequently,  it  was  found  better  first  to  freeze  the  material  and  to  grind  it 
afterwards.  In  the  earlier  tests  this  preliminary  freezing  was  carried  out  in  large  test 
tubes  immersed  in  a  mixture  of  salt  and  ice  at  a  temperature  of  from  -  12°  to  — 17°  < '. 
sometimes  during  cold  weather  the  material  was  placed  out  of  doors  overnight  for  the 
preliminary  freezing.  In  the  remainder  of  the  work  it  was  frozen  by  carbon  dioxide 
and  ether.  Carbon  dioxide  was  obtained  in  the  solid  state  by  allowing  the  compressed 
gas  to  escape  from  the  supply  cylinder  into  a  small  cloth  bag.  The  material  to  be 
frozen  was  placed  in  a  beaker  and  completely  covered  with  solid  carbon  dioxide. 
A  small  amount  of  ether  was  then  added,  until  complete,  freezing  had  taken  place.  A 
temperature  of  approximately  -120°  C.  may  be  obtained  in  this  way. 

The  tissue  is  reduced  to  a  finely  divided  condition  by  grating  or  grinding  in  a  food 
grinder.  The  ground  material  must  be  quickly  and  thoroughly  mixed  before  sam- 
pling, since  as  would  he  expected,  and  as  has  indeed  been  found  by  other  investigators, 
not  all  parts  of  a  given  organ  give  the  same  concentration  of  sap.  Unless  great  care 
is  taken  m  mixing,  two  or  more  samples  of  the  same  pulp  do  not  have  the  same  osmo' 
concentration. 

nil  pie-  are  placed  in  the  freezing  tubes  and  allowed  to  thaw  completely  before 

the  determination  of  the  freezing  point  is  made.     When  the  tissue  is  ground  and 

II  iefore  the  preliminary  freezing  many  serious  changes  may  take  pla 

•  he  liberated,  many  new  chemical  reactions  may  be  brought  about,  and  the 

solutions  may  change  in  various  physical  ways.     After  thawing,  the  material  should 


ve,  J  W    The  freezing-points  of  Tottingham's  nutrient  solution.    Plant  World  17  14. 

Lewis,  G.  N.,  The  osmotic  pressure  of  concentrated  solutions  and  the  laws  of  tl  olution. 

Jour.  Amer.  Chem.  Soe.  30:  668-683,  1908. 

tahinlrriSVT'Ard,R"A"G0rtner'    Noteson  the  calculation  of  the  osmotic  pi  j  of  expressed  vege- 

^Psfro^  *e  depression  of  the  freezing  point,  with  a  tal  the  values  of  II  for 

■ '  <  ■    Am.  Jour.  Bot.  1:  75-78,  1914. 

taSrr!^ttn;nnreUSi0nt0(5f9°C,onalr,,>  '"  determi™  0»  osmoti  ot  expressed  v. 

table  sapsjfram  the  d  »n  of  the  freezing  point.     Amer.  Jour.  Bot.  2:  418  119,1915. 


RESEARCH  METHODS  IN  STUDY  OF  FOREST  ENVIRONMENT.      109 

be  stirred  with  a  glass  or  suitable  wooden  rod  to  expel  all  air  bubbles.  Thawing  may 
be  completed  within  15  or  20  minutes  at  most,  and  the  possibility  of  chemical  change 
is  thus  very  greatly  reduced.  When  thawing  is  complete,  -the  thermometer  is  inserted, 
the  tube  is  placed  in  the  freezing  mixture,  and  the  material  allowed  to  reach  a  tem- 
perature about  1°  below  its  freezing  point.  Solidification  is  then  brought  about,  as 
has  been  stated,  by  turning  the  thermometer  backward  and  forward  a  few  times  to 
create  a  slight  disturbance  in  the  pulp.  It  has  been  found  in  practice  that  much  more 
.satisfactory  results  are  obtained  if  the  material  is  thus  allowed  to  undercool  about  1° 
than  when  solidification  is  brought  about  with  less  undercooling.  In  the  latter  case 
the  mercury  rises  to  the  freezing  point  much  more  slowly  and  the  determination  of 
this  point  is  consequently  more  difficult. 

Osmotic  pressure  in  soils. 

Although  it  is  possible  to  remove  the  soil  solution  from  the  soil 
and  to  determine  its  osmotic  pressure  by  the  freezing-point  method, 
this  will  fall  far  short  of  the  desired  end,  which  is  to  determine  how 
the  water  behaves  in  the  presence  of  the  capillary  forces  and  ad- 
sorption tendencies  of  the  soil  particles  and  colloids.  As  has  been 
suggested  in  the  introductory  paragraphs  to  this  chapter,  these  in- 
fluences may  run  parallel  with  the  influences  of  dissolved  salts  in 
the  soil  water. 

The  freezing-point  determinations  for  moist  soils  are  so  similar  in 
method  to  those  for  plant  pulps  that  it  seems  unnecessary  to  describe 
them  here  in  detail.  The  reader  is  especially  referred  to  the  descrip- 
tion given  by  Bouyoucos  (107) .  It  would  seem  that  the  fundamental 
consideration  in  testing  a  given  soil  at  various  moisture  contents  is 
to  have  samples  very  evenly  wetted.  This  is  accomplished  by  placing 
the  samples  in  a  moisture-tight  jar  and,  after  thorough  shaking,  allow- 
ing it  to  stand  one  or  more  days,  so  that  the  moisture  is  evenly  dis- 
tributed and, has  ample  opportunity  to  be  adsorbed.  While  it  is 
possible  to  use  measured  amounts  of  water  in  wetting  the  soil,  it  is 
probably  safer  procedure  to  take  moisture  samples  at  the  same  time 
that  samples  are  taken  from  the  jar  for  freezing  tests. 

Vapor  transfer  in  soils. — The  vapor  transfer  method  is  the  only 
other  method  of  osmotic  determination  which  appears  feasible  for 
soils,  and  where  time  is  not  an  important  element,  it  is  believed  to  be 
preferable  to  the  freezing-point  method  because  of  the  possibility 
of  treating  soils  in  their  natural  states.  It  should  be  understood,  how- 
ever, that  the  value  of  the  method  is  as  yet  theoretical  rather  than 
proven. 

The  work  of  Alway  (103)  and  Hilgard  (125)  on  the  hygroscopic 
coefficient  of  soils  has  already  been  mentioned,  with  the  suggestion 
that  since  the  moisture  boxes  used  could  not  completely  prevent  the 
vapor  from  escaping,  their  results  are  not  indicative  of  those  to  be 
expected  when  vapor  pressures  are  maintained  in  true  equilibrium 


110  BULLETIN    1059,    U.    S.    DEPARTMENT    OF   AGRICULTURE. 

with  solutions  of  reasonable  osmotic  pressure.  As  has  been  state.!. 
a  solution  showing  12  atmospheres  pressure  will  be  in  equilibrium  with 
vapor  99  per  cent  saturated,  and  even  this  degree  of  saturation,  it  is 
believed,  would  be  extremely  difficult  to  maintain  except  in  a  fully 

sealed  vessel. 

On  the  other  hand,  Patten  and  Gallagher  (136)  have  carried  out 
experiments,  both  on  absorption  of  vapor  and  evaporation  from  soils, 
iu  " desiccators ,;  which  are  assumed  to  be  similar  to  the  bell  jars 
mentioned  hereafter,  and  which,  while  usually  not  strictly  vapor 
proof,  approach  much  nearer  to  the  ideal  condition.  Pat  ten  and 
Gallagher,  both  in  their  review  of  earlier  work  and  in  their  own 
experiments,  have  established  a  number  of  salient  points  which  assist 
in  the  proper  conception  of  the  relation  between  vapor  (or,  to  a  cer- 
tain extent,  gas)  molecules  and  solid  particles,  such  as  those  of  the 
soil.  Schiibler17  and  Davy17  are  quoted  as  having  shown  that,  in 
general,  the  finer  the  texture  of  the  soil  and  the  greater  its  content 
of  humus,  the  higher  is  the  absorption  capacity  of  soil  for  water 
vapor.  These  results,  while  actually  referring  to  the  initial  rate  of 
absorption,  are  fairly  indicative  of  the  forces  with  which  various 
soils  attract  water  vapor.  Von  Dobeneck17  obtained  similar  results, 
though  concluding  that  large  grains  absorbed  more  vapor  per  unit 
of  surface  than  small  ones.  Each  soil  particle  reacts  upon  vapor 
molecules  independently,  and  each  has  a  specific  relation  to  different 
kinds  of  gases.  Several  investigators  have  decided  that  the  absorp- 
tion of  vapor  decreases  with  an  increase  in  temperature,  even  though 
the  absolute  vapor  pressure  increases  proportionately.  Patten  and 
Gallagher  have  carefully  proven  this.  Hilgard's  contrary  finding 
may  be  explained  on  the  basis  that  he  was  dealing  almost  wholly 
with  rate  of  absorption,  and  higher  absolute  vapor  pressure  should 
more  quickly  bring  about  equilibrium.  Mason  and  Richards  17  found 
that  cotton  fiber  containing  water  resembles  a  solution  in  exhibiting 
a  definite  partial  vapor  pressure. 

Patten  and  Gallegher's  most  important  results  have  to  do  with  the 
rates  of  absorption  of  vapor,  and  with  evaporation,  in  the  presence 
of  various  vapor  pressures  controlled  by  sulphuric  acid  solutions 
and  vessels  of  water,  within  desiccators.  The  rate  of  absorption  by 
dry  soils  increases,  and  the  rate  of  evaporation  from  wet  soils  de- 
creases quite  regularly  as  the  partial  vapor  pressure  in  the  desiccator 
is  increased.  It  is,  however,  evident  in  all  of  the  results  that  as  the 
vapor  pressure  approaches  saturation  the  amount  of  absorption  in- 
creases in  greater  proportion  than  does  the  vapor  pressure.  A  num- 
ber of  the  graphs  are  strongly  suggestive  of  the  idea  that,  if  complete 
vapor  saturation  were  attained,  the  absorption  by  the  ^oil>  mighl 
be  unlimited. 


7  For  complete  Citations  see  Bureau  of  Soils.  IVpartment  of  A  griculture,  BulM 


RESEARCH   METHODS  IX   STUDY  OE  EOREST  ENVIRONMENT.       Ill 

With  partial  vapor  pressures  over  acid  solutions,  practical  equi- 
librium was  reached  in  all  soils  at  the  same  ultimate  moisture  con- 
tent, whether  the  soil  was  started  in  a  moist  or  dry  condition.  Thus 
sea-island  cotton  soil,  which  was  the  finest  used,  dried  out  at  a  vapor 
pressure  of  17.90  millimeters  (76  per  cent  of  saturation)  in  97  days 
from  55  per  cent  to  6.08  per  cent  moisture,  and  the  same  soil  ab- 
sorbed in  the  same  period  5.6  per  cent,  starting  from  a  dry  condition. 
In  the  presence  of  a  vessel  of  water  the  drying  out  was  always  very 
slow,  and  the  fact  that  any  drying  whatever  occurred  is  believed  to 
be  sufficient  evidence  that  the  atmospheres  in  the  desiccator  were  not 
saturated,  owing  to  the  presence  of  the  dry  soils  in  the  same  at- 
mospheres. 

Finally,  the  energy  effects  of  absorption  must  not  be  overlooked. 
Patten  and  Gallegher  cite  a  number  of  investigations  which  show 
that  the  heat  released  when  vapor  is  absorbed  by  a  soil  is  in  excess 
of  the  latent  heat  which  is  released  when  vapor  condenses.  This 
fact  indicates  that  water  held  in  the  soil,  like  water  held  in  a  solu- 
tion, is  brought  to  a  greater  density  than  that  in  which  only  water 
molecules  are  attracting  each  other.  This  density  can  only  be  ob- 
tained through  the  release  of  additional  energy. 

Examined  kinetically,  then,  the  whole  situation  is  fairly  simple. 
Molecules  of  a  gas  or  vapor  repel  one  another,  and  this  repulsion 
increases  with  the  temperature  and  energy  of  each  molecule.  When 
a  certain  density  is  obtained  in  a  volume  of  vapor,  the  so-called 
saturation  density,  the  molecules  may  either  return  to  the  liquid 
from  which  they  emanated  or  be  compelled  to  unite  with  other 
molecules,  starting  condensation  in  the  molecular  sense.  In  the  case 
of  atmospheric  vapor,  solid  particles,  such  as  dust  particles,  may 
start  condensation  through  their  attraction  for  vapor  molecules, 
which  latter  would  otherwise  repel  one  another  too  strongly  to  be 
brought  together. 

The  same  phenomena  occur  in  the  soil.  A  soil  particle  of  given 
size,  mass,  and  gravitational  power  can  attract  to  itself  a  certain 
number  of  vapor  molecules,  this  number  depending  upon  the  space 
available  and  the  distance  at  which  the  vapor-molecules  begin  to  repel 
one  another,  or  the  temperature  and  energy  of  these  molecules.  A 
vapor  molecule  which  has  been  trapped,  and  has  given  up  some  of 
its  energy  in  this  process  of  " individual  condensation,'1  is  relatively 
inert,  but  not  so  insert  as  the  soil  particle,  and  is  still  capable  of  re- 
pelling other  molecules  to  some  extent.  The  ultimate  number  of 
molecules  that  can  be  held  in  a  given  soil,  therefore,  must  depend 
(1)  primarily  on  the  energy  of  the  free  molecules  as  governed  by 
temperature;   (2)   on  the  area  or  surface  of  soil  particles  exposed, 


112  BULLETIN  1050,   U.   S.   DEPARTMENT   OF    AGRICULTURE. 


SUCJ 


h  that  vapor  molecules  remaining  on  said  surfaces  do  not  crowd 
one  another:  (3)  on  the  size  of  the  particles,  the  smaller  particles 
having  less  gravitational  power;  and  (4)  on  the  extent  to  which  the 
substances  in  the  soil  act  as  solids  or  crystals  with  only  exterior  sur- 
faces available,  as  spongy  masses  capable  of  obsorption,  or  as  indi- 
vidual molecules  each  of  which  may  attract  and  retain  under  partial 
control  a  sufficient  number  of  molecules,  so  that  in  the  aggregate  the 
conditions  are  those  of  a  liquid. 

The  factors  that  affect  the  rate  of  absorption  are  far  less  impor- 
tant, hut  might  be  briefly  mentioned,  as  follows:  (1)  The  size  of 
3oil  spaces  as  affecting  free  passage  of  vapor  molecules:  (2)  the 
number  of  chances  for  a  given  molecule  in  motion  through  a  given 
space  to  encounter  an  attractive  force  too  strong  for  it  (this  has  to 
do  with  the  number  of  particles  per  unit  of  volume,  as  well  as  size 
<>f  air  spaces);  (3)  the  density  of  the  soil  particles;  (4)  the  density 
of  the  vapor  molecules  as  affected  by  tempera  t  lire  and  whole  pressure; 
5)  the  conductivity  of  the  soil,  governing  the  rate  at  which  the 
heat  of  condensation  can  be  eliminated  from  the  soil  mass. 

This  examination  of  established  facts  and  theory  regarding  vapor 
condensation  in  soils  leads  to  the  recent  efforts  by  Bates  (105)  to 
show  that  the  moisture  of  soils  does  exhibit  a  definite  partial  vapor 
pressure  corresponding  to  that  of  a  solution,  and  that  the  vapor 
transfer  method  has  many  latent  possibilities,  li  should  he  stated 
thai  these  investigations  are  not  yet  complete  or  convincing,  but  in 
some  respects  they  have  gone  farther  than  any  other-  and  an-  worth 
mentioning  at  least  as  suggestions  for  further  effort. 

The  vapor-transfer  method  of  Bates. — In  it-  simplest  form  the 
vapor  transfer  method  is  similar  to  the  plasmolytic  method  in  at- 
tempting to  find  a  point  of  osmotic  equilibrium  between  the  -oil 
moisture  and  solutions  of  known  concentration.  In  this  case  equi- 
librium  must  be  shown  by  the  cessation  of  transfer  of  water,  through 
vapor,  from  the  soil  to  the  solution,  or  the  reverse.  It  would  he 
possible  to  take  a  number  of  samples  of  a  given  soil  at  a  known-mois- 
ture content  and  place  them  in  relatively  small  chambers,  each  with 
a  solution  of  different  concentration  from  the  other-,  and  in  the 
case  in  which  there  was  no  transfer  from  the  soil  to  the  solution  or 
viee  versa  equilibrium  would  exist  and  the  osmotic  pressure  of  the 
soil  water  could  be  directly  calculated  from  the  known  concentration 
of  the  solution. 

In  practice,  however,  it  is  far  more  feasible  to  place  the  soil  -ample 
of  known  moisture  content  in  the  vapor  of  a  solution  of  approxir 
mately  the  same  osmotic  pressure,  let  the  .wo  come  int..  equilibrium 
through  vapor  transfers,  then  compute  the  moisture  content 
the  soil,  the  osmotic  pressure  of  the  solution,  and  the  approximate 
osmotic  pressure  of  the  soil  «»n   the   assumption    that    the 


original 


RESEARCH  METHODS  IX  STUDY  OF  FOREST  ENVIRONMENT.      113 

osmotic  pressure  varies  inversely  as  the    moisture    content.     The 
latter  is  probably  true  only  within  rather  narrow  limits. 

Further,  since  obtaining  an  equilibrium  by  vapor  transfer  is  a 
slow  process,  it  is  desirable  to  be  able  to  treat  a  large  number  of  soil 
samples  simultaneously.  For  this  purpose  a  large-size  bell  jar, 
resting  upon  plate  glass,  may  be  employed.  A  pressure  cooker,  the 
cover  joint  being  properly  sealed,  has  also  been  found  very  useful. 
There  may  be  one  or  more  vessels  of  the  solution,  and  as  many  ves- 
sels of  soil  as  are  desired,  within  the  chamber.  Using  2£-inch  soil 
cans  for  soil  containers  and  similar  beakers  for  solutions,  about 
60  soil  samples  may  be  treated  at  once  under  a  14  by  12  inch  bell 
jar.  However,  a  great  deal  of  evidence  shows  the  desirability  of 
a  relatively  large  vessel  for  the  control  solutions,  and  a  decrease 
in  the  number  of  soil  samples. 

It  should  be  remembered  that  this  treatment  will  only  give  the 
osmotic  pressure  for  each  soil  at  one  particular  moisture  content, 
which  will  depend  upon  the  total  amount  of  water  in  all  of  the 
samples,  as  well  as  upon  the  concentration  of  the  control  solution 
at  the  initiation  of  the  test.  To  obtain  a  range  of  values  for  any 
soil,  separate  tests  must  be  made  under  varying  conditions. 

The  fundamental  provision  in  such  a  test  is  that  the  vapor  cham- 
ber should  have  a  constant  temperature  and  be  evenly  heated  on 
all  sides.  To  accomplish  this  most  simply  a  deep  excavation  in  the 
ground  is  desirable.  The  very  gradual  seasonal  change  of  tempera- 
ture in  such  a  situation  will  not  work  any  harm,  since  the  vapor 
pressure  within  the  chamber  will  adjust  itself  to  such  a  change  with- 
out necessitating  any  condensation.  In  the  lack  of  this,  a  dark 
cellar  may  be  chosen. 

In  proof  of  the  theory  that  a  soil  solution  would  continue  to  ab- 
sorb water  indefinitely  in  the  presence  of  saturated  water  vapor, 
and  that,  therefore,  the  hygroscopic  coefficient  as  now  known  is  a 
purely  empiric  quantity,  a  test  has  been  conducted  for  slightly  more 
than  a  year  with  clean  sand  and  various  modifications  thereof  which 
represent  the  different  elements  encountered  in  various  types  of 
soil.  The  sands  and  modified  sands  were  all  palced  in  the  vapor 
chamber  in  an  oven-dry  state,  together  with  a  bottle  of  distilled 
water  with  linen  wicks  having  about  10  square  inches  evaporating 
surface.  The  successive  weighings  of  samples  and  bottle  indicate 
that  some  vapor  is  constantly  escaping  from  the  chamber,  so  that 
the  vapor  therein  is  never  absolutely  saturated.  Early  in  the  test 
it  was  found  impossible  to  heat  the  chamber  as  evenly  as  desired 
in  the  cellar  in  which  it  was  placed,  so  that  it  was  removed  for  a 
time  to  an  electrically  heated  incubator.  Here  the  fluctuations, 
though  small,  were  rapid  and  a  slight  overheating  at  one  time  caused 
a  very  severe  loss  from  all  samples,  the  chamber  being  unable  to 
i  <  1 1  < ;:  5 — 22 — i ;  1 1 1  i  i  or>n a 


114  BULLETIN   1059,   TJ.   S.   DEPARTMENT   OF   AGRICULTURE. 

bold  the  vapor  under  high  pressure.  The  effect  of  this  is  seen  in 
diagram  7.  where  some  of  the  typical  results  are  illustrated.  In 
Table  5  is  shown  only  the  absorption  for  four  periods,  eliminating 
this  period  of  general  loss.     Except  this  period,  the  exposure  in  the 


DIAGRAM    7 
MOISTURE   ABSORPTION    IN  VAPOR 

BY    FINE  SAND  AND  OTHER  SUBSTANCES 
,,                                                 WITH    FINE  SAND 

'* 

s 

X 

10  u 

1) 

• 

- 

■«6- 

J    ° 

c 
n: 
x: 
u 

• 

• 

V 
ID 

a) 

i_ 

3 

iff'' 

/ 

*-e- 

o 
7  ir. 

L 
<U 
0 

- 

-V 

• 

„  - 

,'-'1 

a 

b 

/' 

• 
s 

■*■  *" 

*  o 

+- 

CL 

5    1_ 

1_ 

• 

s 

>5''' 

■^ 

XI 

4  < 

o 

01 

f 

/ 
/ 

/ 

,,'' 

a) 

in 

/ 
/ 

/ 

- 

to 

5 
t2 

;;^' 

\  5 

/ 
/ 
/ 
/ 

• 

'*" 

^ 

^ 

^^^ 

/ 
/ 
/ 

,' 

45 

»  Hun 

1US 

r 

5lTf  v 

Clav" 
nc  S 

and— 



■ 



" 

^^^ 

_^  — 



""To' 

A  V,F, 

Sand 

.       f, 

?.     1 

5        6 

0         6 

3          1 

00         1 

20         \ 

40         I 

60 

Tim 
80       2 

§6^ 

s 

20        2 

40         Z 

eo      2 

80        J 

00       J 

:         it*        *                   * . ' 

cellar  has  been  at  temperatures  between  0°  and  10°  C,  which,   it 
will  be  noted,  are  lower  than  have  previously  been  employed. 

For  several  of  the  soil  types  duplicates  were  run.  The  results, 
however,  are  so  nearly  identical  in  each  pair  that  only  the  averages 
need  be  given. 


Tabi  e  5.    -Absorption  of  water  vapor  by  Nebraska  fine  sand,  and  modifications  thereof. 

[Basis  100  grams  dry  matter  in  2J-inch  aluminum  can.] 


Xuni- 

ol 

sam- 
ples. 


Description  of  sample. 


Absorption  al  en  1  of— 


> 
days. 


I  if)  per  cent  fine  sand,  unwashed 

100  per  cent  fine  sand,  unwashed  half-size » 

100  per  cent  fine  sand,  washed  2 

90  per  cent  fine  sand,  10  per  cent  very  fine  sand 
0  per  cent  fine  sand,  10  per  cent  silt  and  clav 

80  percent  fine,  lOpercent  very  fine,  10  per  cent  silt  and  clav 
0  per  cent  fine  sand.  10  per  cent  fine  limestone  soil 
">  per  cent  fine  sand,  10  per  cent  calcium  carbonate 
>er  cent  fine  washed  sand,  1  per  cent  K.XO,  3 

98  per  cent  fine  washed  sand,  2  per  cent  K.V  >■■ 

"'  percenl  fine  washed  sand,  3  per  cent  K\'<>> 

98  per  cent  fine  sand,  2  per  cent  ground  decayed  wood'. 

yb  per  cent  fine  sand,  4  per  cent  ground  decayed  wood 


/'</■  i(  at. 
0.56 
.lis 
.  .">7 
.51 
,'.i7 
.93 
.72 
.71 

• :' 

.  73 

.  ^7 

1.15 


10 

da 

days. 

l'ir  ci  nt. 
0.58 

Percent. 
0.68 
.72 

1.10 

L.07 

.  85 
.89 

1.07 

1 .  33 


61 
1.33 
1.37 

1 .  55 

3.01 

1.13 
1.64 


i  w'lSTIdt?'1'1  l)f  volu7e1atl !  ,1"l"h  on  rate  of  absorption. 
\\  ashed  with  5  volumes  of  distilled  wa  I 

otassium  nitrate  applied  in  solution,  an  1  water  ■  -  iporatej  |  irl  Lng  the  test 


sa 
days. 


1.08 

1.58 

1.72 
1.27 
2  05 

l  91 

3 


RESEARCH    METHODS  IX  STUDY  OF  FOREST  ENVIRONMENT.      115 

It  should  be  borne  in  mind  that  the  ideal  temperature  conditions 
were  not  attained  in  this  test,  and,  as  has  been  stated,  that  at  no  time 
has  complete  saturation  of  the  vapor  existed,  except  possibly  for 
short  periods  during  cooling.  This  ideal  has,  however,  probably  been 
approached  more  closely  than  in  any  previous  test,  and  the  long 
period  employed  gives  us  a  new  insight  into  the  phenomena  of 
absorption. 

The  following  comments  on  Table  5  may  assist  in  an  understand- 
ing of  these  results.  The  comparative  behavior  of  various  soil  com- 
binations will  not  be  discussed,  as  these  merely  substantiate  the  ob- 
servations of  others. 

1.  The  amount  absorbed  by  the  unwashed  fine  sand  in  382  days 
is  only  one-third  more  than  the  absorption  in  5  days.  It  is,  however, 
evident  that  even  at  the  end  of  the  longer  period  the  unwashed  sand 
was  not  in  an  atmosphere  of  saturated  vapor,  but  rather  in  one  whose 
pressure  was  quite  as  much  controlled  by  the  presence  of  soils  still 
absorbing  vapor,  and  particularly  by  the  sample  containing  the 
largest  amount  of  potassium  nitrate.  Assuming  that  all  the  moisture 
absorbed  by  the  last  entered  into  the  salt  solution,  the  latter  would 
be  a  22  per  cent  solution  and  would  stand  for  an  osmotic  pressure  of 
more  than  45  atmospheres.  It  is  therefore  not  surprising  to  find  that 
a  soil  which  contains  not  over  20  parts  per  million  of  soluble  matter 
should  make  little  gain  in  this  atmosphere. 

2.  On  the  other  hand,  the  amount  absorbed  by  the  fine  sand  in  382 
days  is  just  about  equal  to  the  wilting  coefficient  for  this  sand,  as 
nearly  as  can  be  estimated  from  a  test  on  the  original  soil,  which 
contained  about  equal  proportions  of  material  finer  and  coarser  than 
the  fine  sand. 

3.  The  continued  and  relatively  large  absorption,  especially  by 
the  soils  containing  active  salts,  might  be  ascribed  to  the  low  tem- 
peratures under  which  the  test  was  conducted.  It  is  believed,  how- 
ever, that  the  evidence  of  a  condition  slowly  approaching  saturation 
vapor  pressure,  and  never  quite  up  to  it,  is  convincing,  and  that  this 
explains  not  only  the  present  results  but  nearly  all  the  phenomena 
that  has  been  reported  in  a  similar  connection. 

A  number  of  other  tests  somewhat  similar  to  the  above  were  made 
during  1918,  but  for  short  periods  only.  Several  attempts  were 
made  to  compare  the  osmotic  pressures  of  soil  samples  in  their 
natural  moisture  conditions,  by  placing  the  fresh  samples  under  a 
single  bell  jar  without  a  control  solution,  to  note  whether  the  samples 
gained  or  lost  moisture  in  the  common  atmosphere.  While  these 
gains  or  losses  indicated  the  relative  dryness  of  the  several  samples, 
the  tests  were  not  continued  long  enough  to  produce  any  results  of 
value.  It  was  found  that  a  period  of  two  or  three  weeks  was  inade- 
quate to  bring  about  equilibrium  between  the  many  samples,  whose 


llf,         BULLETIN    1059,    V.   S.   DEPARTMENT   OF  AGRICULTURE. 

average  weight  was  about  100  grams.  Especially  was  it  noted  that 
surface  samples  which  were  nearly  air-dry  when  taken  absorbed 
vcrv  little  moisture  in  these  short  periods. 

Finally,  from  November  28,  1918,  to  March  29,  1919.  a  test  was 
made  involving  samples  from  nearly  all  of  the  regular  soil-sampling 
paints  al  the  Fremont  Experiment  Station,  as  well  as  a  miscellane- 
ous  Lot  exemplifying  various  peculiar  characters.  Two  bell  jars 
were  employed,  a  small  dish  of  sodium  hydrate  solution  being  placed 
in  each.  At  the  end  of  the  four-month  period  these  solutions  bad 
nut  absorbed  vapor  to  quite  the  same  extent  in  the  two  jars,  and  in 
neither  case  was  the  total  absorption  equal  to  the  losses  from  all  of 
the  soils.     However,  for  practical  purposes  the  two  container-  were 


DIAGRAM   8 
OSMOTIC  AND  CAPILLARY   RELATIONS 

SOILS  FROM  DIFFERENT  DEPTHS   AT    COMMON    POINT 

STA       F  - 1 

Granite  Gravel 

j 

i 1 

:; 

g 

O 

£ 

o 

: 

i/1 

■+- 

c 

if 

16 

> 

> 

o 

UJ 

f 

. 

^ 

\^^ 

> 
a 

lO 

L 

E 

VI       _     _ 

jj2> 

Q 

• 

c 

c 
a 

o 

2 

c 

p 

3 

^•^ 

*- 

3 

- 

a 

\ 

O 

S 
S" 

y 

/ 

i 
1 

1^ 

-' 

>'--- 

^  — 

3 
-4- 

6   5 

■oO 

c 

V 

S * 

,rf> 

__^^r 

„-^ 

^1 

-— ' 

I 

2 

4- 

■*'■ 

si 

'^S^ 

-S*- 

**■- 

*- 

—  " 

—  " 

o 

CO 

£, 

£, — 

^ 

x— ~* 

s"^ 

- 

■ 

6 

1 

3 

So 

3 

1  Mo 

sfur 

e   Pe 

rcerv 

-?     J 

ages 

*       ' 

5 

?            ' 

• 

'              J 

? s 

. 

in  equilibrium  with  each  other,  the  osmotic  pressures  of  the  solu- 
tions being  21.5  and  20.2  atmospheres,  respectively. 

At  the  outset  each  sample  of  soil  was  given  10  per  cent  of  moisture 
above  its  air-dry  weight,  so  that  the  moisture  was  available  in  liquid 
form  and  the  various  soils  were  not  radically  unlike  in  their  initial 
conditions.  While  it  is  not  certain  that  the  time  allowed  was  suffi- 
cient to  establish  equilibrium,  it  is  to  be  noted  that  the  changes  in 
moisture  content  varied  from  losses  of  about  3  per  cent  to  gains  of 
fractional  percentages  and,  in  one  case,  where  there  was  much  raw- 
humus,  a  gain  of  15  per  cent. 

The  results,  as  shown  in  small  part  in  Table  6  and  diagrams  v  «.». 
and  10,  are  very  elucidating.     These  diagrams  are  prepared  some- 


RESEARCH  METHODS  i^T  STUDY  OF  FOREST  ENVIRONMENT.      117 

What  in  the  manner  suggested  for  interpreting  the  moisture  data  for 
soil  wells  and  illustrated  in  diagram  3. 

It  will  be  noted  on  examining  these  diagrams,  each  of  which  rep- 
resents the  soils  from  various  depths  at  a  single  point,  that  in  each 
group  of  soils  of  common  origin  the  lines  drawn  to  connect  the 
capillary  moisture  and  moisture  equivalent  for  each  sample  tend  to 
converge  toward  the  axis  of  the  system  of  coordinates  and  give  the 


5 

26 

24 

DIA15KAM    9 
OSMOTIC  AND  CAPILLARY  RELATIONS 

►OILS  FROM  DIFFERENT  DEPTHS   AT   COMMON    POINT 

STA.     F-Z 

Granite  Gravel 

22 

CD 

20 

o< 

: 

CO 

18 

^ 

(0 

<V 
16    as 

-12     £ 
C    C 

eg   > 

> 

3 

,<£ 

^ 

SL 

4- 

c 

i,Tu 

3 

r 
j 

^ 

y 

y 

y 
y* 

Q_ 

12    <D 

3  c 

I 

>&< 

/         . 

y 

y 

=5 
O 

8    ^ 

o 

-  .  . ..   n) 

-l2 

5         jr 

/ 

-"  ^ 

/ 

d 

^o 

O 
O 

-           W 

» 

^" 

/' 

\ 

o 

6    (0 

c 

^ 

,*€ 

*■ 

'""" 

4_ 

5 

//  / 

V 

O^ 

2 

/* 

/ 

/      2 

4 

/ 

6 

8 

1 

Soi 
0      .' 

1    Mc 

2            1 

isti 

4         I 

ire 

Perc 

enta 

ges 

l 2, 

i           I 

6          2 

3 

suggestion  that  in  any  such  group  of  samples  these  two  measures 
will  vary  proportionately.  On  the  contrary,  there  is  a  decided  tend- 
ency toward  parallelism  in  the  lines  connecting,  for  each  soil,  the 
wilting  coefficient  and  the  moisture  content  at  the  point  of  osmotic 
equilibrium  established  approximately  by  this  particular  test.  If, 
for  example,  the  last-mentioned  moisture  contents,  which  may  be 
termed  the  "  osmotic  equivalents,"  be  taken  to  correspond  in  every 
case  to  20  atmospheres  osmotic  pressure,  and  if  these  be  represented 


118         BULLETIN   1059,    U.    S.   DEPARTMENT   OF   AGRICULTURE. 

by  M'.  and  the  excess  over  wilting  coefficient  by   K,   then   in   these 
diagrams  the  condition  is  represented  by 


rather  than 


W  =  WG+  K 
M'=  WCxK, 


since  it  is  readily  seen  that  the  osmotic  equivalents  are  not  propor- 
tionate to  the  coefficients. 

Table  6.     Osmotic  equivalent  of  soils,  in  presence  of  solution  at  20  atmospJieres,  after 

four  months  exposure  and  other  related  properties. 


ion. 

Depth. 

Capillary 

moisture. 

Moisture 
equiva- 
lent, 
100-G. 

Wilting 
coeffi- 
cient. 

Osmotic 
equiva- 
lent. 

Excess 

over 
wilting 
coeffi- 
cient . 

Change  in 
moisture 

during 

transfer. 

Feet. 
fl      

Per  cent. 
11.64 
10.00 
10.96 
22.08 

Per  cent. 
4.29 
3.60 
4.34 
8.25 

Per  cent. 

2.31 
.93 
.76 

2.13 

Pa  a  nt. 

11.19 

8.7 

8.  92 

1  10.06 

Per  cent. 
8.  88 

v.  Hi 
7.93 
8,  L3 

Grams. 
+0.01 

2 

-2.39 

1-1 

•3 

Well  sand 

-2.35 
1  -0.  72 

■Yv. of  3. 

-1.58 

fl 

11.34 
16.08 
14.84 
24.70 

5.41 
5.91 

5.  S6 
9.29 

1.16 

2.63 
2.67 
3.67 

7.S4 
8.  69 

9.58 
J9.87 

6.68 
6.06 
6.91 
6.20 
6.55 

-2.62 

2 

-2.20 

3 

-1.67 

.    . 

Well  sand 

i  —  1. 24 

Av.  of  3... 

-2. 16 

fl 

23.53 
24.00 
18.86 
21.61 

13.45 

16.25 

11.62 

6.67 

4.72 

3.65 

2.99 

2    .76 

11.06 

10. :: 

10.  2 1 

9.90 

6.34 
7.07 
7.25 

•Mi 

-1.90 

2 

—1.80 

F-ll 

3 

-2.  11 

Well  sand 

—1.29 

Av.  of  3 

-1.94 

1  Average  of  4  samples  taken  from  each  well,  representing  the  surface  and  depths  of  1,  2,  and  3  feet ,  so  1  hat 
mean  value  should  be  equivalent  to  that  of  soil  as  placed  in  the  well. 

\ptiroximate.    Test  made  on  coarse  sandy  soil  from  depth  of  4  feet,  most  nearlv  approaching  the  quality 
•if  sand  used  in  the  well. 

Table  6  shows  that  K  varies  as  between  different  groups  of  samples 
from  different  sources,  but  that  within  a  group  of  similar  origin  K 
is  essentially  a  constant.  Thus,  it  has  an  average  value  of  8.13  per 
cent  for  one  group,  6.55  per  cent  for  another,  and  6.89  per  cent  for  the 
third,  and  this  value  seems  not  to  have  any  constant  relation  to  the 
change  which  occurred  in  the  samples  during  their  period  of  ex- 
posure, so  that  it  may  be  accepted  as  representing  something  near  a 
final  condition.  In  one  sample  representing  a  limestone  soil.  K  is 
found  to  be  17.12-15.33  per  cent,  or  1.79  per  cent.  In  another  soil 
oi  Lava  origin,  containing  less  of  silt  and  clay,  but  a  considerable 
amount  of  sodium  bicarbonate,  K  is  found  to  be  L2.23-4.44  per 
cent,  or  7.79  per  cent. 

These  findings  compel  the  following  conclusions: 

1.  The  wilting  coefficient  of  a  given  soil  is  probably  dependent 
both  on  the  solutes  present  and  upon  the  colloids  capable  of  ad- 
sorbing both  the  solutes  and  the  water,  but  more  particularly  upon 
the  latter;  since  only  very  rarely  will  the  solutes  be  so  abundant 
to  create  an  excessively  strong  solution  before  the  disappearance  of 
the  free  water. 


RESEARCH    METHODS  IN  STUDY  OF  FOREST  ENVIRONMENT.      119 

2.  The  osmotic  equivalent  of  the  soil  is  almost  wholly  dependent 
upon  the  solutes  present,  and  among  soils  of  one  general  type,  in 
which  the  ingredients  are  conducive  to  the  creation  of  solutes  at  a 
definite  rate,  and  their  free  transfer  from  one  point  to  another,  a 
given  osmotic  equivalent  represents  a  fairly  constant  amount  of 
free  water  plus  a  variable  amount  of  unfree  water,  depending  on 
the  quantity  of  clay,  humus,  etc.,  in  each  sample. 

3.  In  such  a  group  of  related  soils  the  wilting  coefficients  may 
have  some  fairly  constant  relation  to  the  capillary  moistures  or  mois- 
ture equivalents,  because  both  measures  are  affected  by  the  water- 
holding  power  of  the  colloids  in  large  part;  but  a  capillary  measure 


DIAGRAM  10 

OSMOTIC   AND  CAPILLARY  RELATIONS 

SOILS   FROM  DIFFERENT  DEPTHS   AT   COMMON    POINT 

STA     F-ll 

Gneiss  Clayey   Sand 

?4 

11 

o 

1 

o 

o 

JS-- 

T-° 

to 

> 

'5 

„-■ 

l^ 

^-*" ' 

V 

18  <0 

.  —  " 

— -" 

£-  • 

^ 

.^■-"" 

> 

„.-£ 

-t- 
c 

o 

S 
*•  • 

,,  — - 

.-  — 

■*" 

,--•' 

--*' 

^- 

-■— ■ 

1A-U-H 

„.- 

-" 



£--" 

3 

4- 

1?" 

o- 

^    -— ■ 

•  -- 

-  — "" 

,  n^; 

y> 

"d 

0 

12^  _< 

£r- 

-'1 

<_> 

0 

V) 
8      i>C 

c 

V 

-  ^- 

5 

-t-  V- 

^-~- 

^ 

>— 

O 

> 

J  ^ 

+>* 

3 
o 
hi 

if 

2 

L       ^ 

en 
o 

S^  1 

2 

. 

i 

__2 

& 

i 

7 

£ 

5 

2 

in 

Vloisl 

0            1 

ure 
1 L 

Peri 

^tj 

*es 

. i 

& I 

, i 

fi L 

2 2 

fl__£ 

1 52 

of  the  condition  of  the  soil  water  is  not  alone  a  safe  criterion  as  to 
its  osmotic  condition  or  availability  at  points  considerably  above 
the  wilting  coefficient. 

It  is  believed  that  these  conclusions  are  essentially  in  accord  with 
those  of  Bouyoucos  (106)  and  Hoagland  (127),  as  derived  from 
their  study  of  freezing  points  and  osmotic  pressures.  Probably  this 
conception  of  the  factor  affecting  availability  is  of  greatest  value 
in  explaining  the  poor  growing  conditions  of  undrained  soils  and 
the  great  preference  of  trees  for  those  which  are  well  drained.  It  is 
also  of  importance  in  indicating  that  soils  of  closely  related  origin 
may  be  compared,  as  to  their  current  conditions,  on  the  basis  of  the 
amount  of  free  or  available  water  in  each.  This  proposition,  it  will 
be  remembered,  the  writers  were  unable  to  accept  with  reference  to 
soils  of  unrelated  origins,  which  gave  rise  to  the  need  for  this  whole 


120         BULLETIN   1059,   U.   S.    DEPARTMENT   OF   AGRICULTURE. 

(  ninputing  the  coefficient  of  availability . 

In  computing  the  coefficient  of  availability  of  the  moisture  at  a 
given  part  of  a  tree  or  other  plant,  allowance  must  be  made  as  has 
been  said,  not  only  for  the  osmotic  pressures  at  work  in  the  plant 
and  soil,  but  for  the  distance  through  which  these  must  operate, 
and  the  effect  of  gravity  on  the  balance  between  the  two  forces.  As 
previously  suggested,  let 
P  osmotic  pressure  in  the  plant, 
P'  =  opposing  pressure  in  the  soil  water, 

L  =  ^==height  of  plant  in  centimeters  at  point  where  P  is  determined, 
G     weight  of  the  column  of  water  to  be  lifted,  in  atmospheres,  or 

equal  to  h  X  0.00097  atmosphere. 
.LI.  representing  the  coefficient  of  availability,  is  equal  to 

P-P'-G 
h 

1 1  may  then  be  assumed  that  the  foliage  of  the  tree,  at  a  height 
of  30  meters  above  the  ground,  has  been  determined  by  its  freezing- 
point  depression  to  possess  an  osmotic  pressure  (P)  of  25  atmos- 
pheres. 

A  more  complicated  case  may  also  be  considered.  A  soil,  previ- 
ously tested,  is  found  to  possess  an  osmotic  pressure  of  25  atmos- 
pheres at  4  per  cent  moisture  content  and  of  5  atmospheres  at  20  per 
cent  moisture  content,  the  former  being  appreciably  above  the  wilt- 
ing coefficient.  This  soil  is  found  to  be  currently  at  0  per  cent 
moisture  content.     Its  osmotic  pressure  P'  may  then  be  computed 


as 


(25  —  5\ 
2Q_4  )  =  22.5  atmospheres. 

The  formula  for  this  case  then  reads 

AA     25-22.5-3000(0.00097)      -0.41 

AA  = 3000 =      3000=-°-00()1'37- 

The  coefficient  of  availability  being  a  negative  quantity  of  any 
magnitude,  it  is  evident  that  the  part  of  the  tree  which  has  been 
examined  can  not  obtain  water  from  the  soil  unless  (1)  the  moisture 
content  of  the  soil  is  increased,  or  (2)  the  foliage  may  withstand 
tother  drying  and  the  creation  of  a  higher  pressure,  without  injury. 
1  nder  the  conditions  stated  as  to  the  wilting  coefficient  of  this  soil, 
"  is  still  probable  that  the  part  of  the  tree  examined  may  obtain 
water  when  it  attains  a  drier  state. 

In  the  examination  of  a  tree  branch  of  appreciable  length,  it  may 
be  necessary  and  desirable  to  make  an  additional  allowance  in  h  for 
the  horizontal  distance,  as  well  as  the  distance  from  the  ground. 
lins  addition,  however,  would  not  apply  to  the  calculation  ol  G 


RESEARCH  METHODS  IX  STUDY  OF  FOREST  ENVIRONMENT.       121 

Other  Soil  Properties  to  be  Studied, 
acidity  axd  alkalinity. 

While  neither  extreme  acidity  nor  alkalinity  is  often  encountered 
in  forest  soils,  because  of  their  usually  good  drainage,  yet  the  subject 
is  one  that  should  not  be  overlooked,  even  though,  on  account 
of  its  relative  unimportance,  it  must  be  given  rather  scant  space. 
Unfortunatelv  because  of  deficiencies  in  chemistrv  itself  and  a  lack 
of  proper  understanding  of  the  method  by  which  the  activity  of  acids 
in  the  soil  might  be  measured,  reliable  results  in  such  measurements 
bearing  on  problems  of  plant  distribution  are  only  just  beginning  to 
appear;  for  this  reason,  it  is  unsafe  to  say  that  the  concentration 
of  acids  in  the  soil  either  is  or  is  not  an  ecological  problem  distinct 
from  the  moisture-supply  problems  which  have  just  been  described. 
The  suggestion  of  direct  toxicity  of  soluble  substances  in  the  soil  is 
frequently  encountered,  but  so  far  as  known  no  one  has  shown  that 
toxic  effects  are  not  effects  produced  by  the  cessation  of  the  water 
stream.  It  has  also  been  frequently  suggested  that  active  acids  or 
alkalis  in  the  soil  combine  to  withhold  from  the  plant  the  substances 
needed  for  its  nutrition.  This  seems  more  probable.  Skepticism  in 
these  matters  is  designed  primarily  to  indicate  that  such  questions 
are  still  open  to  investigation  from  more  than  one  angle.  The 
methods  for  determining  acidity  and  alkalinity  in  soils  will  be  briefly 
reviewed,  as  though  these  were  matters  entirely  independent  of  the 
subject  of  water  supply. 

A  recent  and  readily  grasped  article  by  Wherry  (141)  is  filled 
with  good  suggestions  on  the  vexed  question  of  measuring  the  acidity 
of  soils,  and  should  be  read  by  everyone  who  intends  to  go  further 
with  this  discussion.  Among  his  suggestions,  an  outline  given  by 
him  to  cover  the  various  methods  of  acidity  measurement  will  be 
followed,  with  some  elaboration,  also  bringing  up  at  appropriate 
points  the  corresponding  methods  applicable  to  the  determination  of 
alkalinity.  It  should,  perhaps,  be  explained  that  the  term  "  alka- 
linity' is  here  used  in  its  chemical  sense,  and  not  with  the  broader 
meaning,  sometimes  permitted,  of  total  soluble  salts. 

1.  A  salt  solution  is  added  to  the  soil.  For  this  purpose  there  haAre  been  used 
sodium  chloride,  potassium  chloride  and  nitrate,  calcium  chloride,  nitrate  and  acetate, 
zinc  sulphide  plus  calcium  chloride,  etc.  The  quantity  of  acid  in  the  resulting  solu- 
tion, which  represents  that  originally  present  in  the  soil  plus  a  much  greater  amount 
produced  indirectly  by  the  processes  18  outlined  is  then  determined  by  titration  or 
other  means. 

In  the  appendix  to  this  paper  has  been  given  in  detail  the  titra- 
tion method  for  acidity  following  the  "  extraction ':   of  the  acids  of 

18  Briefly,  replacement  of  H-ions  in  compounds  which  would  in  stable  condition  show  no  evidence  of 
the  weak  acids  present. 


122  BULLETIN    L059,    U.    S.    DEPARTMENT   OF   AGRICULTURE. 

the  soil  by  potassium  nitrate  solution,   a  method  which  has  been 

much  used  and  debated,  but  which  should  probably  from   now  on 

be  considered  only  for  its  historical  interest.     It  has  many  times 

been  found  that  this  method  produces  appreciable  acidity  in  soils 

which  at  the  same  time  evidence  alkalinity. 

2.   \<>  salt  solution,,  but  some  pure  water,  is  added  to  the  soil. 

The  mixture  is  titrated  with  lime  water,  using  either  an  indicator  or  observation 
of  the  freezing  point  to  determine  the  end  point.  This  gives  the  amount  of  lime 
nee  led  to  neutralize  the  acid  originally  present  in  the  soil  plus  thatproduced  indirectly 
by  the  action  of  lime  (which  is  likely  to  differ  from  that  produced  by  a  neutral  Ball 
solution  i  as  well  as  the  amount  of  lime  required  to  satisfy  the  absorptive  power  of  the 
soil  colloids  tor  calcium  ion  under  the  given  conditions. 

The  determination  of  the  end  point  in  such  a  water  mixture  by 
the  freezing-point  method  is  the  method  described  by  Bouyoucos 
IDS),  and  is  based  on  the  fact  that  as  long  as  the  CaOH  added  is 
combining  with  a  free  acid  or  an  acid  salt  (which  is  up  to  the  point 
of  neutrality),  the  solution  will  contain  fewer  and  fewer  ions,  and 
consequently  will  have  a  higher  and  higher  freezing  point.  When 
the  CaOH  molecules  begin  to  remain  in  solution,  however,  there  is 
an  immediate  change  in  the  opposite  direction.  This  method  appears 
to  have  considerable  value,  though  not  wholly  a  measure  of  the  free 
acids.  Likewise,  when  the  reaction  of  the  solution  lias  been  shown 
not  to  be  acid,  through  an  immediate  lowering  of  the  freezing  point 
on  adding  CaOH,  it  would  seem  that  the  normal  freezing-point  de- 
pression was  a  measure  of  the  alkalinity. 

(6)  The  mixture  is  filtered  and  the  filtrate  titrated  with  standard  alkali.     Trvs 
gives  the  quantity  of  acid  present  in  the  soil. 

By  titration  with  KHS04  solution,  the  filtrate  may  likewise  be 
tested  for  alkalinity,  the  method  being  described  in  the  appendix. 
It  is  perhaps  desirable  to  bring  out  here,  however,  since  both  of  these 
methods  may  be  used,  that  the  commonly  used  indicator,  phenol- 
phthalein,  does  not  indicate  neutrality,  but  a  specific  alkalinity  of 
30.  Wherry  suggests  the  use  of  litmus  of  brom-thymol  to  detect 
complete  neutrality. 

(c)  The  hydrogen-ion  concentration  or  specific  acidity  (or  alkalinity  -  is  determined— 

a.  By  catalysis  of  an  ester. 

b.  By  measurement  of  the  potential  due  to  hydrogen-ion  with  the  potentio- 

meter. 

c  By  observation  of  color  changes  of  indicators  whose  relations  to  hydrogen- 
ion  concentration  are  known. 

This  last-named  method  is  that  which  Wherry  then  describes  in 
detail.     It  consists  primarily  of  the  use  of  six  indicators  in  various 


RESEARCH    METHODS  IX  STUDY  OF  FOREST  ENVIRONMENT.       123 

combinations,  such  that  variations  between  a  specific  acidity  of  3,000 
and  a  corresponding  u superalkalinity "  may  be  detected  with  not  too 
great  refinement,  yet  probably  with  all  the  precision  necessary  in 
studying  the  distribution  of  plants.  These  extremes  correspond,  re- 
spectively, to  hydrogen-ion  concentrations  of  PH  =  3.5  and  PH  =  10.5. 
For  the  most  precise  determinations  of  the  degree  of  alkalinity  or 
acidity  the  potentiometer  is  undoubtedly  the  last  word.  A  number 
of  such  instruments  are  on  the  market  and  should  require  practically 
no  adaptation  for  the  treatment  of  soil  extracts.  Xo  reason  appears 
why  they  might  not  be  readily  used  in  the  field.  Apparently  the 
apparatus  devised  by  Briggs  (1 1 1 "»  for  determining  the  ''soluble 
salt  content  of  soils"  w^as  of  very  similar  nature,  though  its  relation 
to  hydrogen-ions  was  probably  little  understood  at  the  time. 

THE    MECHANICAL    ANALYSIS    OF    SOILS. 

A  mechanical  analysis  of  any  soil  which  is  being  studied  exten- 
sively is  probably  worth  while  if  only  to  give  a  convenient  and 
approximately  correct  name  for  the  soil.  Thus  may  be  avoided  the 
<>rror  of  speaking  of  a  soil  as  a  ''clay'  when,  in  fact,  it  contains  80 
per  cent  silt  and  only  a  very  little  clay,  or  perhaps  even  a  large 
component  of  very  fine  sand  and  small  amounts  of  the  finer  mate- 
rials which  make  it  as  stiff  as  clay.  With  accumulated  analyses  of 
-oils,  too,  comparison  will  show  whether  the  mechanical  analysis  of 
two  are  very  similar,  approximately  what  water-holding  capacity  a 
new  soil  may  have,  what  wilting  coefficient,  etc.  However,  in  this 
calculation  the  humus  plays  a  very  important  part  and  its  effect  is 
difficult  to  estimate. 

The  method  of  mechanical  analysis  which  may  be  considered 
standard  has  been  recently  described  by  Fletcher  and  Bryan  (120). 
It  employs  a  number  of  sieves,  with  perforations  of  successively 
smaller  size,  which  separate  the  particles  of  various  sizes  but  allow 
the  very  fine  sand,  silt,  and  clay  to  pass  through.  These  three  grades 
are  then  separated  in  water  under  the  action  of  gravity. 

The  standard  soil  grades  recognized  by  the  Bureau  of  Soils, 
United  States  Department  of  Agriculture,  are  indicated  by  the  fol- 
lowing table  of  diameters  (Table  7) ,  which  also  indicates  the  diam- 
eters of  the  circular  perforations  in  the  standard  sieves.  Opposite 
these  values  have  been  set  the  approximately  corresponding  sizes 
of  screens  which  are  adapted  for  handling  larger  samples  in  the 
study  of  forest  soils,  under  what  may  be  called  the  "English'7  rather 
than  the  metric  system  of  classification. 


124  BULLETIN    1050,    U.    S.    DEPARTMENT    OF    AGRICULTURE. 

Table  7. 




ravel 

.  vol 



Medium  sand 

sand 

line  sand  — 
Silt. 


Metric.  (Diameters  of  sieve  perforations.) 


Millimeters. 


Over  2 

2.00  to  1.00. 
1.00  to  0.50. 
0.50  to  0.25. 


English.  (Number  of  openings  per  inch). 


Less  than  t. 
4  to  10. 
H)  to  20. 
20  to  40. 
40  to  60. 


(1.2.-)  to  0.10 i  60  to  100 


0.  lit  too .05  (setl  les  in  test  tubein30s?conds) 
0.05  to  .005  (settles  in  centrifuge  in  5  min- 
utes at  800  revolutions  per  minute). 
.005  to  0000  (does  not  s?ttle  in  centrifuge; 
measured  by  deduction). 


Over  100  (settles  in  hot  tie  in  30  seconds). 
Same  as  metric. 

Turbid  water  evaporated  and  weighed. 


The  following  procedure  is  suggested  as  the  result  of  a  good  deal 
of  experience  in  treating  forest  soils: 

1.  If  the  soil  to  be  sampled  contains  a  great  deal  of  rock,  say  over 
25  per  cent  by  volume,  and  of  large  size,  it  is  desirable  to  determine 
the  rock  percentage  by  sifting  a  considerable  quantity  of  the  mate- 
rial tl trough  a  screen  having  four  meshes  to  the  inch.  This  should 
be  done  only  when  the  material  is  air-dry.  and  should  be  accom- 
panied by  much  beating  and  brushing  to  remove  the  fine  material 
from  the  rock  surfaces.  After  the  process,  a  sample  of  about  100 
grams  of  the  finer  material  may  be  taken.  If  rocks  are  few  and 
small,  it  is  better  to  sample  and  wash  them  with  the  other  material, 
separating  them  when  dry  from  the  coarse  gravel  on  the  2-milli- 
meter or  10-mesh  sieve. 

2.  The  sample  is  placed  in  a  wide-mouth  8-ounce  bottle,  which  is 
then  nearly  filled  with  clean  tap  water,  stoppered,  and  placed  on  the 
shaking  machine,  or  attached  to  a  pulley  which  is  turning  at  the 
rate  of  about  100  revolutions  per  minute.  The  amount  of  shaking 
necessary  will  vary  from  two  to  eight  hours  with  different  soils,  but 
should  always  be  sufficient  to  break  down  every  lump  of  whatever 
size.  If  the  soil  lacks  gravel  for  its  own  pulverizing,  place  two  or 
three  round  pebbles  in  the  bottle. 

'■J>.  The  lumps  thoroughly  broken  down,  the  contents  of  the  bottle 
are  placed  on  the  coarsest  screen,  with  the  finer  sieves  in  succession 
below  it,  and  the  whole  nest  standing  over  a  can  of  1  or  2  gallons 
capacity.  The  material  is  washed  down  through  each  screen  by  a 
tiny  stream  of  water,  until  all  silt  and  clay  have  been  removed;  thai 
is,  until  the  water  comes  through  perfectly  clear.  The  not  of  >ie\  es 
may  then  be  placed  in  the  oven  to  dry.  after  which  the  separation  of 
the  sands  is  readily  accomplished  by  a  little  jarring  of  each  sieve; 
the  material  held  on  each  is  weighed  promptly,  before  it  can  take  up 
moisture  from  the  air. 

4.  The  very  fine  sand  which  passes  the  sieves  after  drying  is  placed 
in  the  washing  bottle.  The  water  from  the  washing  of  the  material 
several  hours  earlier  may  now  be  decanted  off  into  a  measuring  vessel, 


RESEARCH  METHODS  IN  STUDY  OF  FOREST  ENVIRONMENT.      125 

leaving  the  very  fine  sand  in  the  can,  some  silt  and  clay,  and  a  little 
water.  The  material  is  also  transferred  to  the  washing  bottle. 
As  the  first  measure  of  liquid  in  the  bottle  will  be  very  rich  in  silt 
and  clay  at  least  one  minute  should  be  allowed  for  the  very  fine 
sand  to  settle.  After  this  time  the  silt  and  clay  are  partially  de- 
canted into  the  measuring  vessel.  More  water  is  added  to  the  bottle 
and  is  thoroughly  btirred.  With  each  successive  washing  the  time 
is  reduced,  so  that  as  the  water  becomes  nearly  clear  the  sand  is 
allowed  just  30  seconds  to  settle  through  a  4-inch  column  of  water. 
It  will  be  noted  that  the  settling  is  somewhat  slower  if  the  water  is 
extremely  cold. 

5.  All  the  very  fine  sand  is  now  in  the  wash  bottle,  in  which  it  may 
be  dried  and  weighed,  and  all  of  the  silt  and  clav,  with  a  considerable 
volume  of  water,  in  the  measuring  vessel.  It  will  be  economical  to 
obtain  the  weights  of  the  silt  and  clay  by  merely  sampling  this  large 
volume  after  thorough  stirring.  Perhaps  100  cubic  centimeters  may 
be  drawn  off  for  centrifuging  from  a  total  volume  of  2  liters.  The 
amount  and  fineness  of  the  material  thrown  down  in  the  centrifuge 
will  depend  on  the  time  of  centrifuging  and  the  speed  of  the  machine. 
These  should  be  adjusted  after  repeated  trial  and  examinations  of 
the  suspended  particles  under  the  microscope.  (See  Briggs,  Martin, 
and  Pearce  (117).)  However,  as  the  standards  for  "clay,,:  "silt/ 
etc.,  are  purely  arbitrary  any  investigator  may,  for  his  particular  pur- 
poses, adopt  his  own,  as  by  deciding  on  a  period  of  centrifuging 
which  will  in  every  case  clear  the  water  of  particles  of  visible  size. 

The  centrifuging  completed,  the  clay  water  is  decanted  off  into 
one  evaporating  dish,  and  the  silt  in  each  tube  is  washed  out  with  a 
fine  jet  of  water  into  another.  These  are  dried  in  the  oven.  Care 
should  be  used  to  avoid  weighing  either  the  clean  dishes  or  dishes 
containing  tins  fine  material  when  the  general  humidity  is  very 
high.  The  amount  of  silt  and  clay  in  the  evaporators  having  been 
determined,  the  total  amount  for  the  whole  sample  is  readily  cal- 
culated. 

6.  The  quantities  have  now  been  determined  in  nine  grades,  and 
the  percentage  of  the  whole  which  each  grade  represents  may  be 
readily  computed.  The  several  percentages  may  be  entered  on  the 
form  for  u Summary  of  Physical  and  Chemical  Properties  of  Soil" 
(p.  134). 

It  will  be  noted  in  the  following  key  that  no  grade  coarser  than 
coarse  sand  is  mentioned.  In  analyses  made  by  the  Bureau  of  Soils 
it  is  customary  to  pass  the  material  through  the  2-millimeter  sieve 
before  sampling  and  to  base  all  calculation  on  the  total  weight  of 
this  ''fine  earth";  that  is,  material  not  coarser  than  fine  gravel.  In 
forest  soils  coarser  material  is  too  commonly  met  with  to  be  ignored, 
and  its  importance  from  certain  points  of  view  may  be  as  great  as 


l->6  BULLETIN    1059,    U.    S.    DEPARTMENT    OF   AGRICULTURE. 

that  of  the  soil  proper.  It  is  believed,  however,  to  be  desirable  to 
describe  the  soil  in  such  manner  as  to  denote  separately  the  presem  . 
of  a  coarse  matrix  and  a  finer  soil  occupying  its  interstices.  Thus 
if  rocks  or  gravel  formed  more  than  10  per  cent  of  the  mass  we  might 
speak  of  the  soil  as  a  " rocky  medium  sand"  or  a  "gravelly  loam. " 
In  this  event  the  fine  gravel  and  finer  material  should  be  considered 
constituting  100  per  cent  when  using  the  following  key: 

I  LASSIFICATION    OF    SOILS    ON    MECHANICAL    ANALYSIS. 

Soilfi  containing  -20  silt  and  clay: 

l  oarse  sand 25+  very  coarse  sand  and  coarse  -and 

and  less  than  50  any  other  grade. 
,,,1 25  +  very  coarse  sand,  coarse  and  me- 
dium   sand,    and    less    than    50    fine 
sand. 

Fine  sand 50+  fine  sand,  or  —25  very  coarse  sand. 

coarse  and  medium  .-and. 

Very  line  sand 50+  very  fine  sand. 

Soils  containing  20  to  50  silt  and  clay: 

Sandy  loam 25+    very    coarse    -and.     coarse    and 

medium  sand. 

Fine  sandy  loam 50+  fine  sand  or  —25  very  coarse  aand, 

coarse  and  medium  sand. 

Sandy  clay —20  silt. 

Soils  containing  50+  silt  and  clay: 

Loam —20  clay,  —50  silt. 

Silt  loam -20  clay,  50+  silt. 

«  lay  loam 20  to  30  clay,  —50  silt. 

Sihy  clay  loam 20  to  30  day.  50+  silt. 

<  lay 30+  clay. 

THE    DETERMINATION    OF    HUMUS 

The  amount  of  humus  in  the  soil,  which  plays  an  important  part  in 
the  water  relations  and  may  also  be  an  important  source  of  nutrient-, 
may  be  determined  in  two  general  wa\  - 

1.  By  ignition,  taking  no  account  of  the  degree  of  decomposition 
of  the  organic  matter,  and  always  involving  seme  error  through  the 
evaporation  of  water  which  may  exist  in  several  forms  in  oven-dried 
soils. 

"-).  By  extraction  of  the  humified  portion  of  the  organic  matter 
with  ammonia,  and  its  subsequent  ignition. 

It  should  be  realized  that  these  two  methods  produce  entirely 
different  results  and,  in  fact,  they  have  distinct  purposes.  On  the 
one  hand,  the  total  organic  matter  is  of  interest  because  of  it-  bear- 
ing on  the  water-holding  properties  of  the  soil,  and  m  this  connection 
the  total  loss  on  ignition  is  probably  as  expressive  a-  any  other  meas 
ure,  though  m  soils  containing  large  quantities  "I"  carbonates  some 
correction  must  be  made  for  their  breakdown.     It    is,   however,   a 


RESEARCH  METHODS  IX  STUDY  OF  FOREST  ENVIRONMENT.      127 

misnomer  to  call  this  a  "  humus  determination."  On  the  other  handr 
the  amount  of  humified  material  is  important  as  a  possible  source  of 
nitrogen,  it  being,  according  to  Hilgard  (125),  " wholly  uncertain  to 
what  extent  the  unhumified  material  will  ultimately  become  humus, 
from  the  nitrification  of  which  plants  are  presumed  to  chiefly  derive 
their  nitrogen." 

Loss  on  ignition. 

Loss  on  ignition,  as  has  been  said,  may  be  of  interest  in  connection 
with  water-holding  properties.  It  is  readily  determined  by  placing 
approximately  100  grams  of  the  soil  in  a  shallow  earthen  or  platinum 
dish,  in  which  it  will  first  be  oven-dried  and  weighed  and  then  heated 
to  red  heat  in  a  gasoline  or  electric  oven,  with  a  moderate  current  of 
air  pressing  over  it.  Providing  lumps  have  been  broken  down  at  the 
outset,  the  oxidation  may  usually  be  completed  in  an  hour.  After 
this  the  sample  is  again  weighed,  the  ignition  loss  is  calculated,  and 
the  percentage  of  loss  is  based  on  the  oven-dry  weight  of  the  sample- 

In  the  case  of  soils  containing  considerable  lime  or  magnesium  car- 
bonate, the  error  through  the  breaking  down  of  these  on  ignition  may 
be  largely  eliminated  by  a  preliminary  treatment  with  dilute  hydro- 
chloric acid,  as  in  the  humus  extraction  method. 

The  ammonia-soluble  humus. 

The  ammonia-soluble  humus,  or  matiere  noire  of  Grandeau  (122) 
is  the  aim  of  all  of  the  more  recent  methods  of  extraction.  Lime  and 
magnesia  are  first  removed  by  washing  the  soil  with  dilute  hydro- 
chloric acid.  Grandeau  mixed  the  washed  soil  with  coarse  sand 
and  placed  it  in  a  funnel  at  the  bottom  of  which  were  fragments  of 
glass  or  porcelain.  The  whole  mass  was  then  moistened  with  dilute 
ammonia  and  allowed  to  digest  for  three  or  four  hours,  after  which 
the  solution  was  washed  through  with  water  or  water  containing  a 
little  ammonia.  The  filtrate  was  then  evaporated  to  dryness, weighed,, 
ignited  in  a  platinum  dish,  and  weighed  again.  The  loss  on  ignition 
is  the  measure  of  the  extractable  humus.  The  residue  is  termed 
humus  ash. 

Hilgard  19  modified  the  Grandeau  method  by  placing  the  soil  in  a 
paper  filter,  covering  it  with  a  disk  of  filter  paper,  and  here  perform- 
ing both  the  acid  washing  and  the  ammoniacal  extraction.  The  latter 
is  accomplished  with  4  per  cent  ammonia  water  until  the  filtrate  comes 
through  colorless. 

Others  have  attempted  to  improve  on  Hilgard's  method  in  order 
to  expedite  the  process;  but,  as  shown  by  Alway,  Files,  and  Pinckney 
(101),  they  have  introduced  serious  error  through  including  in  the 
ammoniacal  extract  considerable  amounts  of  colloidal  clav  which,  on 

19  In  Bulletin  3X,  Bureau  of  Chemistry,  United  States  Department  of  Agriculture,  1893. 


]'28  BULLETIN    1059,   U.   S.   DEPARTMENT   OF  AGRICULTURE. 

ignition,  suffers  a  loss  of  water.  The  experience  of  the  writers  in- 
dicates that  the  sand  filter  devised  by  Grandeau,  with  also  the  paper 
filter  used  by  Hilgard,  will  come  nearest  holding  the  colloids  in  the 

il.  Such  as  are  likely  to  escape  will  have  passed  the  filter  by  the 
time  the  acid  t  reatment  is  complete.  In  the  case  of  the  coarser  forest 
soils  the  addition  of  sand  is  wholly  unnecessary. 

Alwav  suggests  the  recording  of  the  humus  ash  percentage  as 
well  as  the  humus  as  a  means  of  detecting  the  errors  which  commonly 
enter  into  this  determination. 

CAPILLARY   CONDUCTIVITY. 

As  has  been  frequently  pointed  out  in  the  discussion  oi  the  mois- 
ture problems,  the  rate  at  which  a  plant  is  able  to  obtain  water  from 
the  soil  particles  with  which  the  roots  are  in  actual  contact  may  have 
an  important  bearing  on  the  wilting  coefficient  for  t  he  whole  soil  mass, 
and  may,  in  turn,  depend  largely  on  the  facility  witli  which  the  mois- 
ture travels  from  one  soil  particle  to  another  when  there  is  unequal 
distribution.  Thus,  a  clean  sand  is  generally  understood  to  have  the 
highest  conductivity  (whether  because  of  the  close  contact  between 
the  particles  or  because  of  the  clearness  of  the  spaces  between  the 
larger  particles,  is  not  known),  while  clay  in  the  soil  seems  to  impede 
this  movement,  probably  because  of  absorption,  and  humus  appears 
to  retard  the  movement,  possibly  by  breaking  the  contacts  between 
the  mineral  particles. 

The  whole  subject  of  capillary  conductivity  appears  to  have  been 
thrown  into  confusion  in  recent  years  by  practical  findings,  especially 
in  connection  with  the  study  of  moisture  supplies  in  the  arid  farming 
regions  of  the  West.  In  brief,  it  has  been  found  that  in  certain  lo- 
calities the  soil  is  never  moistened  to  a  greater  depth  than,  say,  10 
feet,  by  precipitation;  that  the  moisture  which  goes  beyond  the 
depth  of  ordinary  crop  roots  is  never  brought  toward  the  surface  by 
capillary  action,  and  hence  is  lost  for  practical  purposes;  that  fallow- 
ing with  the  object  of  storing  moisture  in  the  deep  soil  is  therefore 
useless. 

Buckingham  (116)  and  McLaughlin  (132)  have  apparently  made 
the  most  exhaustive  studies  of  the  movement  of  soil  moisture:  and 
it  may  be  said  that  these  investigations  confirm  the  practical  con- 
clusion that  when  the  mean  moisture  content  is  very  Low  and  the 
difference  in  moisture  between  two  points  is  slight,  the  rate  of  mot 
ment  from  the  moister  to  the  drier  point  is  negligible.  These,  of 
course,  are  the  conditions  to  be  dealt  witli  as  the  wilting  coefficient 
is  approached,  and  it  is  somewhat  relevant  to  remark  that  there 
is  no  evidence  against  the  ordinary  conception  of  capillary  movi 
ment  when  the  amount  of  moisture  in  the  soil  is  considerable,  h  La 
true,  however,  that  this  movement  is  very  slow  upward-  that  Ls, 
against  the  force  of  gravity. 


RESEARCH  METHODS  IN  STUDY  OF  FOREST  ENVIRONMENT.       129 

Considering  only  that  which  is  particularly  relevant  to  the  prob- 
lem, it  was  found  by  Buckingham  that  considerably  different  types 
of  soil  show  about  the  same  capillary  flow  under  the  same  condi- 
tions. A  moist  and  a  dry  layer  of  soil  were,  in  these  experiments, 
placed  one  above  the  other  in  direct  contact.  The  amount  transferred 
from  the  moister  to  the  drier  soil  in  a  given  time  was  found  to  depend 
almost  wholly  on  their  original  difference  in  moisture  content;  fur- 
thermore, the  greater  part  of  this  transfer  was  accomplished  in  a 
very  short  time. 

A  great  many  methods  have  been  devised  for  showing  the  rate  of 
movement  of  water  in  soils;  but  none  of  these,  so  far  as  known,  is 
readily  standardized  or  will  produce  closely  comparable  results  on 
duplication  with  the  same  soil.  This  is  because  the  granulation  of 
soil  has  an  important  influence  on  the  capillary  forces  set  up.  In 
view  of  this  difficulty,  no  procedure  can  be  suggested  which  is  more 
likely  to  produce  reliable  comparative  data  than  the  following: 

After  the  completion  of  the  moisture  equivalent  determination  on 
the  centrifugal  machine,  all  water  having  been  extracted  which  is 
subject  to  the  force  employed  (whether  this  be  100  gravity  or  3,000 
gravity),  and  the  soil  being  then  in  a  state  of  compactness  which  is 
somewhat  close  to  a  standard,  add  to  the  unit  volume  of  soil  a  small 
standard  amount  of  water,  say  10  cubic  centimeters;  as  soon  as  this 
has  been  absorbed  in  the  surface  centrifuge  again  for  short  periods 
until  the  amount  added  has  been  extracted,  determining  the  time  for 
this  unit  process.  This  should  be  a  measure  of  the  resistance  offered 
by  the  soil  to  the  passage  of  a  unit  amount  of  water  through  a  unit 
distance  (the  distance  may  be  somewhat  variable,  but  correction  may 
be  made  directly) . 

It  would  be  unwise  to  leave  this  very  open  subject  without  refer- 
ence to  the  possibilities  of  the  electrical  conductivity  method;  for, 
as  Buckingham  (116)  has  shown,  there  is  a  close  correlation  between 
the  conditions  affecting  electrical  conductivity  and  those  affecting 
water  conductivity  in  a  given  soil.  It  would  seem  that  there  is  also 
a  chance  for  correlation  between  heat  conductivity  and  water  con- 
ductivity. 

CHEMICAL   ANALYSIS    FOR    NUTRIENTS. 

It  may  be  said  that  almost  nothing  is  known  as  to  the  quantitative 
requirements  of  most  plants  for  the  nutrient  materials  obtainable 
from  the  soil  with  the  soil  water,  and  little  enough  as  to  the  elements 
which  in  greater  or  less  quantity  are  essential  for  growth.  The  lack 
of  knowledge  with  respect  to  trees  is  especially  glaring,20  little  atten- 
tion having  been  paid  to  the  subject  because  of  the  almost  universal 
belief  that  the  requirements  of  trees  are  satisfied  by  almost  any  soil. 

20  The  writers  do  not  consider  the  evidence  obtained  by  the  examination  of  leaf  ash  and  other  similarly 
crude  methods  even  as  convincing  evidence  of  qualitative  requirements. 


130         BULLETIN   1050,   U.   S.   DEPARTMENT   OF   AGRICULTURE. 


That  this  is  probably  true  of  such  trees  as  the  pines  is  evidenced  by 
their  adherence  to  light,  sandy  soils.  In  fact,  rather  low  require- 
ments may  be  assumed  for  all  of  the  evergreens  on  theoretical  grounds, 
because  of  the  fact  that  the  green,  functioning  parts  are  of  long  life 
and  the  main  product,  cellulose,  is  a  purely  organic  compound. 

ologically  speaking,  evidence  of  any  direct  part  played  by  soil 
fertility  in  the  distribution  of  species,  and  especially  of  forest  trees, 
rarely  found.  This  may  be  partly  explained  by  the  fact  that 
forest  soils  are  usually  young  and  potentially  fertile,  so  that  other 
characteristics,  especially  water-holding  capacity,  come  into  greater 
prominence.  Much  careful  work  must  be  done,  however,  to  deter- 
mine where  and  when  soil  fertility  becomes  an  important  ecological 

factor. 

Much  difference  of  opinion  exists  as  to  how  the  fertility  of  the 
soil  should  be  measured.  There  is  potential  fertility  in  practically 
all  of  the  soil  mass  except  the  silicon,  and  actual  available  fertility 
only  in  those  substances  which  are  currently  in  solution  with  the 
soil  water.  As  has  been  pointed  out,  notably  by  Hoagland  (127) ,  the 
quantity  of  all  substances  in  solution  varies  not  only  with  the  drain 
upon  these  substances  by  plants,  but  with  the  quantity  of  the  soil 
water.  For  practical  purposes  these  substances  may  be  said  to  be 
soluble  only  to  a  limited  extent. 

The  ordinary  complete  quantitative  analysis  of  a  soil  involves  the 
treatment  of  all  of  the  mass  susceptible  to  chemical  action,  with  a 
view  to  discerning  potential  fertility.  In  some  young  soils,  espe- 
cially if  formed  in  situ,  these  potentialities  may  be  arrived  at  by  the 
experienced  person  through  examination  of  the  mother  rock.  Where 
there  is  any  question,  however,  the  ordinary  investigator,  because  of 
the  great  amount  of  equipment  and  technique  involved,  should  refer 
samples  for  analysis  to  some  well-equipped  laboratory,  such  as  that 
of  the  Bureau  of  Soils  in  Washington.  Four  or  five  pounds  of  the 
soil  are  required  for  complete  analysis.  The  samples  should  be  thor- 
oughly air-dried  when  taken  from  the  ground,  freed  of  rocks,  and 
shipped  either  in  jars  or  in  heavy  canvas  sacks  from  which  the  fine 
material  will  not  be  lost. 

To  obtain  a  measure  of  the  total  soluble  salts  readily  available  in 
the  soil  solution,  where  the  chemical  make-up  of  the  soil  is  gen- 
erally known,  or  may  be  assumed  to  be  adequate  for  all  needs,  ex- 
traction of  the  solutes  by  leaching  may  be  employed.  For  com- 
parative purposes,  the  amount  which  may  be  extracted  with  five 

■lumes  of  distilled  water  (1  liter  for  200  grams  of  soil)  will  serve 
as  well  as  a  more  thorough  extraction.     The  soil  is  placed  on  a   . 
paper  filter,  in  a  6-inch  funnel,  and  the  water  is  poured  on  to  it,  a 
few  cubic  centimeters  at  a  time,  through  a  period  of  24  hours.     Be- 


RESEARCH  METHODS  IN  STUDY  OF  EOREST  ENVIRONMENT.      131 

fore  the  soil  has  thoroughly  settled,  some  clay  is  likely  to  pass 
through  the  filter.  This  is  eliminated  by  pouring  on  to  the  soil  a 
second  time  the  first  100  cubic  centimeters  of  water  which  passes 
through.  When  all  of  the  water  has  drained  out,  the  solution  may 
be  partly  boiled  away  and  allowed  to  cool,  when  the  suspended 
matter  will  largely  flocculate  and  may  be  removed  by  a  second  fil- 
tering. The  clear  solution  is  then  evaporated  in  a  weighed  por- 
celain dish.  Solutes  varying  in  amount  from  20  to  1,500  parts  per 
million  of  the  soil  weight  are  ordinarily  found  in  such  extracts. 

This  subject  has  been  investigated  in  great  detail  by  King  (129). 

For  the  purpose  of  ecology,  qualiative  analyses  showing  the  pres- 
ence in  some  quantity  of  the  elements  and  compounds  known  to  be 
essential,  may  often  be  all  the  chemical  evidence  that  is  required  to 
throw  the  burden  upon  some  other  environmental  condition.  Fol- 
lowing Osborn  (135),  who  has  rather  recently  given  a  summary  of 
the  evidence  on  this  subject,  the  investigator  may  look  for: 

1.  Nitrogen  (in  the  form  of  nitrates),  as  an  essential  constituent 
of  protoplasm,  required  in  large  quantities  when  the  proteins  are 
being  produced,  as  in  seed  formation.  Nitrogen  is  itself  practically 
useless  without  nitrifying  agencies  in  the  soil,  so  that  the  presence 
of  humus  is  not  absolute  proof  of  the  abundance  of  nitrates.  Ni- 
trogen in  certain  forms  may  also,  as  shown  by  Schreiner  and  Skinner 
(138),  inhibit  plant  growth.     This  is  a  subject  of  great  complexity. 

2.  Phosphorus,  as  an  essential  of  the  nuclei  of  cells. 

3.  Iron,  as  an  essential  of  protoplasm,  and  playing  an  important 
part  in  the  formation  of  chlorophyll.  A  lack  of  iron  in  available 
form  is  quickly  shown  in  yellowing  or  "chlorosis"  of  foilage. 

4.  Magnesium,  as  a  constituent  of  the  chloroplasts. 

5.  Sulphur,  required  for  forming  proteins. 

6.  Potassium,  probably  as  a  regulator  of  life  phenomena  through 
chemical  reactions. 

7.  Chlorine,  commonly  present  in  plants  and  probably  functioning 
in  metabolism. 

It  is  with  a  view  to  detecting  the  lack  of  some  of  these  substances 
that  the  following  simple  tests  are  enumerated,  requiring  the  mini- 
mum of  laboratory  equipment  and  technical  skill. 

The  sample  of  air-dried  soil  which  is  to  be  examined  should  be 
placed  in  a  glass  jar  and  distilled  water  added  to  the  amount  of  five 
to  eight  times  the  volume  of  the  soil.  After  about  five  minutes  the 
solution  may  be  used. 

One  hundred  cubic  centimeters  of  the  solution  may  be  tested 
qualitatively  for  chlorine.  For  this  purpose,  to  the  soil  solution 
should  be  added  one  or  two  drops  of  potassium  chromate  (K2Cr204) 
solution  and  titrated  from  a  dropper  by  a  weak  solution  of  silver 


13 'J  BULLETIN   1059,    U.    S.   DEPARTMENT   OF   AGRICULTURE. 

nitrate  AgN03).  If  chlorine  is  present  it  will  be  precipitated  as 
silver  chloride.  The  test  for  chlorine  can  be  made  also  more  simply 
by  taking  some  of  the  soil  solution  in  a  test  tube  and  adding  silver 

nitrate  solution. 

For  of  the  presence  of  other  chemical  substances  which  may 

be  of  importance  in  the  life  of  plants,  the  following  procedure  is 

ygested : 

ree  hundred  cubic  centimeters  of  the  soil  solution  are  poured 
into  a  porcelain  dish  and  slowly  evaporated,  the  drying  being  con- 
tinued almost  to  red  heat  in  order  to  burn  any  organic  matter.  If 
the  organic  matter  is  not  burned  up  in  the  porcelain  dish  contain- 
ing the  dry  residue,  aqua  regia  is  added  and  the  liquid  evaporated 
dryness  at  least  twice.  The  residue  is  then  heated  to  red  heat  in 
order  to  render  the  silicic  acid  insoluble.  The  soluble  residue  is 
dissolved  by  heating  in  a  weak  solution  of  hydrochloric  acid,  and 
the  liquid  is  filtered  off  from  the  while  amorphous  residue  of  the 
aeid.  The  filtrate  is  collected  into  a  graduate  and  water  is  added 
to  bring  it  up  to  300  cubic  centimeters.  Of  this  the  following 
amounts  are  taken  for  further  tests: 

1.  One  hundred  cubic  centimeters  for  determining  the  entire 
amount  of  Fe203  (ferric  oxide) +P205  (phosphorus  pentoxide) + 
A1203  (aluminum  oxide)  +CaO  (calcium  oxide) +MgO  (magnesium 
oxide). 

The  solution  is  neutralized  with  sodium  carbonate  \a,CO:j/) 
until  some  cloudiness  appears,  then  from  5  to  10  cubic  centimeters 
of  sodium  acetate  (NaC2H302)  are  added  and  the  solution  is  heated; 
the  entire  amount  of  ferric  oxide  and  aluminum  oxide  will  be  pre- 
cipitated in  the  form  of  basic  acetates.  These  are  filtered  off.  The 
filtrate  is  heated  and  neutralized  with  ammonia  (NH3)  until  an 
alkaline  reaction  is  obtained,  then  1  or  2  cubic  centimeters  of  am- 
monium chloride  (NH4C1)  and  ammonium  oxalate  (NH4)2  (COO)2 
are  added.     The  calcium  is  precipitated  and  is  filtered  off. 

To  the  filtrate,  after  it  has  been  cooled  off,  are  added  several 
cubic  centimeters  of  sodium  ammonium  phosphate  (NH4XaIIP(),), 
and  it  is  left  to  stand  for  several  hours.  If  magnesium  is  present 
it  will  be  precipitated. 

2.  One  hundred  cubic  centimeters  for  determining  80,   (sulphuJ 

trioxide). 

For  this  determination  the  100  cubic  centimeters  are  heated  and 
to  the  solution  are  added  several  drops  of  barium  chloride  (BaCl2). 
If  S03  is  present,  it  should  precipitate  in  the  form  of  barium  sul- 
phate (BaS04).  If  it  is  not  precipitated  at  once,  the  solution  to 
winch  barium  chloride  has  been  added  is  left  to  stand  for  several 
hours. 


RESEARCH  METHODS  IX  STUDY  OF  FOREST  ENVIRONMENT.      133 

3.  One  hundred  eubic  centimeters  for  determining  P205  (phos- 
phorus pentoxide) . 

In  order  to  determine  the  presence  of  phosphorus  pentoxide 
(P205),  100  cubic  centimeters  of  the  solution  is  neutralized  with 
ammonia  until  an  odor  is  perceived,  the  solution  is  made  acid  by 
the  addition  of  weak  nitric  acid  (HN03),  and  to  such  acid  solution 
there  is  added  from  10  to  15  cubic  centimeters  of  molybdic  acid 
(H2Mo04) . 

A  qualitative  test  for  ferric  oxide  can  be  made  in  the  water  solu- 
tion of  the  soil  by  hydrochloric  acid.  Some  hydrochloric  acid  is 
added,  together  with  several  drops  of  potassium  sulplio  cyanide 
(KSCX).  A  pink,  or  often  bright  red,  color  will  indicate  the  pres- 
ence of  ferric  oxide  in  the  solution.  When  the  soil  is  rich  in  ferrous 
oxides,  their  presence  can  be  readily  acertained  by  dropping  directly 
upon  the  soil  some  of  the  potassium  ferricyanide  (K3Fe(CN)6)  solu- 
tion which  with  the  ferrous  oxides  gives  a  blue  color. 

The  form  for  the  "Summary  of  Physical  and  Chemical  Properties 
of  Soil'  is  provided  with  a  number  of  blank  spaces  in  which  the 
results  of  qualitative  or  quantitative  tests  for  various  salts  may  be 
entered. 


134         BULLETIN  1059,   U.   S.  DEPARTMENT  OE  AGRICULTURE. 


cn 

g 

o 
to 


- 


to 


o 

CO 

a 

co 


O 

CO 


o 
O 

>> 

© 

ft 

3 
© 
o 
o 

CB 

w 

c3 
ft 


3 
co 


3 
o 

S-, 

O 

a 
o 

ft 


> 

© 


03 

fc« 
CD 

03 


© 

5* 
O 
,fi 
03 

d 
c 

— 
03 

> 
OS 


~      * 


o 

03 
& 
CO 

03 


OS 

ft 

o 

CO 


o 

to 

■g 

CO 


1 

i-H 

as 
,3 
co 


CO 

03 


S 
J. 


o 
.3 
ft 

0. 
T3 
co 

OS 

*j 
e3 
o 

-a 

3 


OS 

A 

OS 

d 


.  o 

Jo 

03 

-  — • 

co  '— 

O  33 

o 

CO 


£§ 

os  d 

CO  — 

t-    CO 

O  3 

mS 

S3  3 

E~ 

*2 
co  d 

.   ~c 

©53 

,0  03 
03  -C 

'£  2 

<—  d 

co  oft 
>>: 


H         CO  j^    J 

is  d*  o 


>. 
■=■ 
Pi 

fc.  S  0 .2  -d 

O  3  a  © 
■  >>>>So3 

-j-.-C'c  2  u  S3 

•g  -H   OS   OS   03   o  -w 

8|33s©s 

Cr  oocddT 

-2  as  -^  u^2 
as  Sid  d  o-f.2 

If  »'»?fSa 

C  -c  d  d  .3  ™  S5 
as  d  o  o  3  o"r 
ftoJ^r  © 
;d:d  c3 


S 


«~  d23  03  £,d 
3  d— '  .5  (2. 


„  txd  d 

-gfe  OS  ©  O  g 
,2 .2  M  M  2  ~  Jo 

75  ex  o3  a  C  o3  S 
— •  o  d  3  *:  ^  d 

08  ft  -3  .3  73  ©  d 

*j  o  cs  o3  -r  3  3 

KCQQWCK 


© 
03 


5» 

© 
T3 


© 
co 
3 

-3 
c 

J3 
© 


OQ 
W 
EH 

5" 

H 
55 
W 

o 
a 

CO 

& 


^ 


- 


© 

CO 

3 

■O 

c 
,d 

© 


>< 
1-1 
< 
Iz: 
< 

o 

— 

55 
«! 
3 
o 


& 


^ 


&5 


© 
« 

.3 

— 
3 
CO 


Other  Data. 

(  Give  Standard  (Bur.  of  Soils)  name  based 

on  Analysis)  (7). 

• 

Total 
weight 
sample 
exam- 
ined. 

6^ 

i*s 

Very  fine 
sand 

*  ■ 

Fine 
sand 

Medium 
sand 

Coarse 

sand 

Fine 

gravel 

Coarse 
gravel 

Rocks 

J3 

ft 

0 

0 

a 

0 

0 

1 

- 

i 

-  — 

— 

— 

1 

RESEARCH  METHODS  IN  STUDY  OF  FOREST  ENVIRONMENT.      135 


c 

«3 

p 


.a 

o 

03 


oo 


s? 


5 


B 
Ah 


Permeability  (13). 

E-i,o 
03 

.2-S 

Total 

time, 

minutes. 

Inch,  rate,  minutes. 

o 

13 

J3 
o 

d 

•** 

-a 

o 

d 

CO 

Deasity 

o 

/— N 
CN 

^H 
^^ 

tH 

0) 
03 

•s 

O 

■— « 

,Q 
03 

03 
> 

O 

. 

O 
CD 

8 

o 

CO 

— » 
03 

p 

*£ 

|g 

Moisture 
(10)  equiv- 
alent %• 

Capillary 
(9)  ca- 
pacity (C 
or  CI)  %. 

Satura- 
tion ca- 
pacity 
(S)%. 

o 
o 
— 

o 

CO 

e 

03 

c 

CD 

P 

a 

c 

= 

v. 

C 

V 

"5 
c 
"B 

C 

<— 
c 

a 

= 

._ 
J- 

C 

— 
1 — 

c 

DC 

~. 

c 
>"S 

'- 
- 

C 

V 

ci 

= 

>*s 

— 
1 — 

s 

15 
c 

'- 

- 

* 

z 

'c 
~f 

1 

.0 

CP 

03 


m 

< 

< 


= 


Insoluble 
matter 

£ 

i£ 

£ 

j£ 

£ 

^ 

Acidity 

1 

1 
1 

Alkalinity  (15) 
/o- 

Total  soluble 
matter  (14) 

p, 

® 

P 

a 
S 
es 

s- 

1 
) 

-  4- 

-  4- 

1  -+- 

1 
1 

136         BULLETIN  1059,   U.   S.  DEPARTMENT   OF  AGRICULTURE. 

Summary  of  Soils  Discussion'. 

The  preceding  discussion  has  attempted  to  bring  out  the  theoreti- 
i  considerations  which  make  the  study  of  soils  very  import  ant  in 
forestry,  from  the  standpoint  of  initiation  of  seedlings,  later  com- 
petition between  individuals  and  species,  and  the  rate  and  ultimate 
limit  of  height  growth  on  any  particular  site.  In  the  main  allother 
■  il  conditions  have  been  considered  in  their  bearing  on  the  supplv 
of  soil  moisture.  In  view  of  the  length  of  the  discussions,  it  would 
appear  desirable  to  repeat  the  salient  points,  as  follows: 

1.  It  is  believed  that  from  every  ecological  aspect  the  important 
soil  condition  is  the  availability  of  the  soil  moisture. 

2.  Plans  for  the  study  of  this  soil  condition  have  been  based  on 
the  assumption  that  the  relation  between  the  plant  and  the  soil  in 
which  it  grows  can  best  be  demonstrated  if,  at  any  time,  the  status 
of  either  may  be  expressed  in  terms  of  osmotic  pressures. 

3.  The  generally  coarse  character  of  forest  soils,  and  the  presence 
of  rocks  which  are  as  characteristic  as  any  other  part  of  the  soil  and 
can  not  properly  be  eliminated,  give  rise  to  the  need  for  special 
methods  of  examining  forest  soils,  and  particularly  for  methods 
adapted  to  larger  samples  of  the  soil  than  have  commonly  been  used 
in  agricultural  investigations. 

4.  The  total  moisture  of  the  soil,  while  not  directly  making  possible 
the  comparison  of  sites,  if  there  be  any  variation  in  soil  composition, 
must  be  had  for  most  of  the  indirect  methods  of  comparison;  and  in 
forest  studies  it  must  be  determined  periodically  through  one  or  more 
seasons  in  order  to  discover  the  conditions  that  are  critical.  The 
quantity  may  sometimes  be  found  through  ordinary  methods  of  sam- 
pling and  drying  the  soil  samples;  but  often,  because  of  mechanical 
difficulties,  and  to  insure  greater  physical  uniformity  in  the  samples 
from  time  to  time,  it  is  desirable  to  have  " wells"  of  prepared  soil 
from  which  successive  samples  will  be  taken. 

5.  If  it  seems  desirable  to  compute  the  moisture  of  the  natural 
soil  from  that  found  in  a  soil  well,  this  may  be  done,  at  least  approxi- 
mately, by  comparison  of  the  capillarities,  moisture  equivalents,  and 
wilting  coefficients  of  well  soil  and  natural  soil,  respectively.  It  seems 
probable,  however,  that  up  to  a  high  moisture  content  osmotic  equi- 
librium is  more  likely  than  capillary  equilibrium  between  the  well 
and  the  natural  soil,  so  that  if  the  moisture  of  the  former  may  be 
expressed  in  terms  of  osmotic  pressures,  it  is  unnecessary  to  compute 
the  moisture  of  the  soil. 

6.  For  any  study  of  the  critical  situations  in  soil  moisture,  either 
for  seedlings  or  for  older  trees,  it  is  necessary  to  know  the  wilting 
coefficient  of  each  soil  under  consideration.  The  moisture  content  al 
which  a  plant  may  wilt,  however,  varies  widely  not  only  according  I 


RESEARCH  METHODS  IN  STUDY  OF  FOREST  ENVIRONMENT.      137 

physical  properties  of  various  soils,  but  also,  for  any  given  soil,  ac- 
cording to  the  manner  in  which  the  test  is  conducted,  the  age  and 
species  of  the  plants  employed,  and,  more  important  still,  according 
to  the  atmospheric  conditions  at  a  time  when  the  moisture  supply  is 
running  low.  In  short,  the  wilting  coefficient  is  dependent  very 
largely  on  the  rate  at  which  the  plant  must  obtain  water  in  order  to 
balance  losses.  As  the  atmospheric  conditions  are  difficult  to  control, 
and  practically  impossible  to  reproduce  from  time  to  time  and  place 
to  place,  it  follows  that  wilting  coefficients  are  empiric  quantities 
and  have  no  precise  value. 

7.  It  is  probably  very  desirable  that  wilting  tests  should  be  con- 
tinued as  a  further  check  upon  theory,  and  for  the  further  establish- 
ment of  relations  between  different  species  and  different  soils.  Rela- 
tive values  for  different  species  and  soils,  of  much  value  and  interest, 
are  to  be  obtained  through  simultaneous  tests  at  any  given  point,  and 
by  such  comparisons  a  scale  of  values  either  for  soils  or  for  species 
may  eventually  be  built  up.  There  are,  hoAvever,  indirect  methods  of 
arriving  at  the  wilting  coefficient  which  are  not  only  desirable  for 
practical  purposes,  but  will  add  greatly  to  our  understanding  of  the 
variations  in  wilting  coefficients  due  to  biological  and  environmental 
factors. 

8.  The  study  of  the  freezing  of  soil  water,  the  study  of  the  ac- 
quirement of  moisture  by  soils  when  exposed  to  saturated  vapor, 
and  even  the  behavior  of  the  soil  water  when  subjected  to  an  ex- 
ternal mechanical  force,  all  point  to  the  fact  that  water  may  exist 
in  the  soil  as  a  liquid,  capable  of  more  or  less  movement  from  one 
soil  particle  to  another,  or  as  a  vapor;  21  that  is,  as  separate  water 
molecules,  held  in  place  by  the  affinity  of  the  solid  particles,  and 
thereby  prevented  from  moving.  All  signs,  too,  point  to  the  fact 
that,  except  possibly  in  soils  of  unusual  alkalinity  or  acidity,  the 
soil  water  is  truly  nonavailable  only  when  it  ceases  to  function 
as  a  liquid.  While  wilting  of  plants  may  often  occur  with  liquid 
water  still  available,  this  is  readily  accounted  for  by  the  slow  rate 
of  movement  toward  the  roots,  which  becomes  a  probability,  espe- 
cially in  clay  and  humous  soils,  whenever  the  volume  of  water  is 
not  large.  Water  obviously  moves  much  more  readily  in  coarse 
than  in  fine  or  humous  soils;  and,  as  has  been  mentioned,  the  rate 
which  may  be  fatal  to  a  plant  depends  on  the  needs  of  the  plant 
as  determined  bv  its  losses. 

9.  While,  therefore,  no  method  has  yet  been  devised  by  which  the 
theoretical  and  exact  wilting  coefficient  may  be  directly  arrived  at, 
any  one  of  the  methods  mentioned  in  the  preceding  paragraph  has  its 

21  It  would,  perhaps,  be  more  descriptive  of  the  kinetic  status  to  speak  of  this  as  "solidified  water"  and 
it  is  not  certain  that  a  wide  separation  of  the  molecules,  as  in  vapor,  is  an  essential  part  of  the  situation. 


138  BULLETIN   1059',   U.    S.   DEPARTMENT   OF   AGRICULTURE. 

possibilities  as  an  avenue  of  approach.  Thus,  by  the  freezing-point 
method  the  osmotic  pressure  of  the  soil  solution  corresponding  to 
any  given  moisture  content  may  be  determined;  and  by  several 
trials,  the  point  at  which  the  soil  water  ceases  to  behave  as  a  liquid, 
thai  is,  ceases  to  show  a  definite  freezing  point,  may  be  determined 
quite  closely.    This  water  content  is  probably  the  term  sought  as  a 

andard.  The  freezing-point  method  has  one  serious  objection, 
namely,  that  it  forbids  keeping  the  soil  during  test  in  a  natural  state 
of  compactness,  or  with  a  natural  arrangement  of  the  soil  particles. 

The  vapor  transfer  method  has  also  many  possibilities.  In  the 
ordinary  determination  of  the  hygroscopic  coefficient  the  water 
vapor  is  probably  not  entirely  saturated;  and,  in  consequence,  under 
certain  empiric  conditions  of  the  test,  there  may  be  obtained  in  a 
limited  time  a  limited  absorption  of  vapor  by  the  soil  which  was 
air-dry  at  the  outset.  The  quantity  absorbed,  so  far  as  the  tests  go, 
hears  a  fairly  constant  relation  to  the  wilting  coefficient,  the  ratio  of 
the  two  (juantities  being  approximately  2:3.  The  objection  to  this 
method  is  that  the  conditions  are  purely  empiric,  and  the  quantity 
of  soil  treated  is  very  small. 

By  exposing  soils  to  water  vapor  in  a  vapor-tight  chamber,  such 
as  a  bell  jar,  and  in  the  presence  of  a  solution  of  an  active  salt  which 
represents  a  given  osmotic  pressure  and  vapor  pressure,  considerable 
masses  of  soil  may,  after  a  long  period,  be  brought  to  vapor  pres- 
sure equilibrium  with  the  solution  and  with  one  another.  The  mois- 
ture content  of  each  soil  is  then  the  "  osmotic  equivalent  "  of  the  con- 
trol solution,  whose  osmotic  pressure  is  readily  calculated.  The 
osmotic  pressure  to  exist  at  the  end  of  the  test  may  be  in  part  con- 
trolled by  calculations  at  the  outset,  and  later  by  changing  the  con- 
trol solution.  A  solution,  at  the  end  of  the  test,  representing  50 
atmospheres  osmotic  pressure,  for  example,  might  be  taken  as  a 
standard  for  establishing  osmotic  equivalents  in  lieu  of  wilting  coeffi- 
cients.  The  objections  to  this  method  are  the  long  time  required  to 
complete  a  test  and  the  absolute  need  for  a  constant  temperature,  or 
at  least  for  the  elimination  of  rapid  changes.  The  former  objection 
is  partly  counterbalanced  by  the  number  of  soils  which  may  be 
treated  simultaneously. 

In  the  moisture-equivalent  determination,  as  so  far  conducted,  the 
moisture  of  any  soil  is  submitted  to  a  definite  centrifugal  force  which 
tends  to  separate  it  from  the  soil.  Within  the  limits  of  agricultural 
types  of  soil,  at  least,  the  force  of  1,000  gravity  employed  by  Briggs 
and  Shantz  (114)  appears  to  leave  in  the  soils  amounts  of  water 
which  bear  a  nearly  constant  relation  to  the  respective  wilting  co- 
efficients. Experiments  with  a  force  of  100  gravity  have  shown 
wide  variations  in  results  with  different  types  of  forest  soils,  indicat- 
ing that,  as  humus  and  clay  proportions  vary,  both  strong  and  weak 


RESEARCH  METHODS  IN  STUDY  OF  FOREST  ENVIRONMENT.      139 

capillary  tensions  are  set  up,  and  these  do  not  react  in  the  same  way 
on  relatively  large  and  very  small  quantities  of  water  in  the  soil. 
It  appears,  therefore,  as  suggested  by  Free  (121),  that  the  effective 
procedure  is  the  employment  of  a  variable  force,  sufficient  in  the 
case  of  any  particular  soil  to  extract  all  of  the  water  which  is  extract- 
able.  It  seems  probable  that  this  quantity  would  correspond  to  all  of 
the  liquid  water  capable  of  moving  from  one  part  of  the  soil  to 
another.  The  remaining  water  would  probably  correspond  closely 
to  the  uunfree  water ■''  of  Bouyoucos  (106)  and  what  in  this  bul- 
letin is  termed  " water  vapor,"  or  water  whose  molecules  are  too 
rigidly  held  by  solid  substances  to  have  the  motility  of  liquid  mole- 
cules. While  this  method  is  as  yet  untried  and  would  obviously  be 
more  laborious  than  the  present  standardized  procedure,  possibly  pre- 
senting new  mechanical  difficulties,  it  promises  so  much  as  a  direct 
and  rapid  means  of  determining  a  physical  constant  that  it  deserves 
serious  investigation.  In  the  meantime  the  1,000-gravity  test  should 
be  employed  as  the  basis  for  comparing  the  moisture  conditions  of 
various  soils  and  in  the  detailed  study  of  their  wilting  coefficients. 

10.  Once  the  wilting  coefficient  of  a  soil  has  been  determined,  di- 
rectly or  indirectly,  the  current  moisture  condition  may  be  expressed 
in  terms  of  the  percentage  of  available  moisture  or  the  available 
moisture  per  unit  of  soil  volume,  by  subtracting  the  nonavailable 
moisture  from  the  whole.  The  amount  of  water  per  unit  of  volume 
recommends  itself  particularly  in  comparing  the  conditions  of  open 
and  dense  forest  stands,  provided  that  the  root  extent  of  the  indi- 
vidual tree  has  been  investigated.  Without  such  information,  very 
wrong  conceptions  of  the  moisture  supplies  available  to  the  indi- 
vidual trees  in  different  forest  types  are  likely  to  be  formed. 

11.  For  the  purpose  of  expressing  the  condition  with  which  any 
individual  plant  is  coping,  particularly  the  conditions  against  which 
a  seedling  must  struggle  in  times  of  drought,  it  is  very  desirable  to 
reduce  the  water  content  of  the  soil  to  terms  of  availability.  If  it 
is  assumed  that  the  wilting  coefficient  stands  for  a  definite  osmotic 
pressure  in  the  soil,  with  which  the  osmotic  pressure  in  the  plant 
is  in  equilibrium  (this,  of  course,  being  only  approximately  true,  as 
pointed  out  in  paragraph  6,  and  being  further  subject  to  the  condi- 
tions of  the  plant,  as  indicated  in  the  following  paragraph),  then, 
when  the  moisture  content  is  equal  to  the  wilting  coefficient,  the 
availability  of  the  soil  water  is  0. 

When  the  moisture  content  of  the  soil  is  twice  as  great  as  the  wilt- 
ing coefficient,  about  twice  the  osmotic  pressure  may  be  expected  in 
the  plant  as  in  the  soil,  and  this  should  make  possible  a  fairly  definite 
rate  of  absorption  by  the  plant.  The  availability  at  this  point  may 
be  called  0.50.  Similarly,  when  the  moisture  content  is  three  times 
the  wilting  coefficient,  the  availability  may  be  expressed  as  0.67.     It 


140         BULLETIN   1059,    U.   S.   DEPARTMENT   OF   AGRICULTURE. 

should  be  understood  that  these  are  arbitrary  terms,  and  thai  thej 
only  express  relatively  the  conditions  under  which  a  given  plant  in  a 
given  soil  may  be  obtaining  its  water  from  time  to  time,  perhaps  a 
little  more  clearly  than  these  conditions  can  be  expressed  through 
the  percentage  of  available  water. 

12.  Finally,  the  plan  of  expressing  the  relation  between  the  plant 
and  it-  moisture  supply  currently  and  accurately  through  the  osmotic 
pressure  of  each  has  been  conceived.  The  difference  in  osmotic  pres- 
sure in  favor  of  the  plant  expresses  the  degree  of  control  of  the 
plant  <>vcr  its  water  supply:  but,  since  the  highesl  osmotic  pressure 
in  the  plant  is  likely  to  be  attained  at  that  point  which  is  farthest 
Prom  the  roots,  where  also  the  danger  is  greatest,  it  is  evident  that 
in  considering  the  availability  of  water  to  this  point  there  must  be 
considered  the  distance  through  which  the  differentia]  pressure  must 
operate;  or,  in  other  words,  the  osmotic  gradient,  say.  per  centimeter 
of  stem  tissue,  etc.  This  gradient  will  also  be  affected  by  gravity. 
A-  the  coefficient  of  availability,  therefore,  a  term  has  been  proposed 
which  brings  these  elements  into  their  proper  relation-  with  definite 
value-.     Thus, 

AA= — i — 

a  formula  which  promises  to  be  especially  enlightening  in  studying 

the  phenomena  of  growth  in  older  tree-,  as  they  compete  with  one 
another  and  reach  their  limits  of  height  for  a  given  site.  The  value 
of  AA  is  seen  to  fluctuate  with  each  change  caused  by  water  loss  or 
accretion  in  the  region  where  P  is  determined,  as  well  as  with  gradual 
changes  in  the  conditions  of  the  soil  moisture. 

13.  The  discussion  has  included  other  aspects  of  the  -nil.  which, 
with  the  possible  exception  of  nutrition,  it  is  believed  should  he 
considered  only  as  indicator  aspects;  that  is,  these  aspects  will  only 
serve  to  explain  the  phenomena  of  soil  moisture.  They  include  al- 
kalinity or  acidity,  humus  content,  composition  'as  indicated  by  the 
sizes  of  the  soil  particles),  and  the  capillary  transporting  power  of 
the  soil. 

11.  In  the  study  of  seedlings  during  their  most  critical  period-  of 
establishment  -this  being  the  period  when  the  character  of  the  plant 
society  is  most  largely  determined— it  is  believed  that  the  percentaj 
of  available  moisture  within  reach  of  the  usually  short  root-  is  of 
primary  importance.  To  determine  this  with  any  accuracy  will  he 
found  difficult  on  account  of  the  usually  heterogeneous  character  of 
the  sod  layer  that  will  be  involved.     The  proper  study  requires: 

(a)  Determination  of  root  depth  at  each  examination.  Tt  is 
through  this  determination  that  a  distinction  between  species  ma\ 
be  made. 


RESEARCH    METHODS  IN  STUDY  OF  FOREST  ENVIRONMENT.      141 

(6)  Selection  of  the  soil  sample  at  the  point  of  maximum  moisture 
content  within  the  zone  reached,  which,  of  course,  in  the  important 
periods  of  drought,  will  usually  be  the  deepest  point  reached. 

(c)  Determination  of  whole  moisture  of  each  sample. 

(d)  Determination  of  moisture  equivalent  of  each  sample,  at  least 
as  a  means  for  classification  of  unlike  samples. 

(e)  Retention  of  samples,  with  determinations  of  wilting  coeffi- 
cients on  typical  classes,  probably  after  the  period  of  field  observa- 
tions. In  these  determinations  it  will  be  well  to  compare  the  be- 
haviors of  the  two  or  more  species  involved. 

It  is  believed  that,  with  tiny  seedlings,  the  quantity  of  water 
usually  required  to  maintain  life  is  so  small  that  the  available  volume 
maybe  left  out  of  consideration.  In  other  words,  the  samples  may 
be  practically  point  samples,  seeking  always  the  maximum  available. 
Even  this  painstaking  examination  of  soil  moisture,  however,  may 
be  futile  without  a  record  of  the  conditions  conducive  to  water  loss, 
particularly  the  temperature  conditions  at  the  surface  of  the  soil. 

15.  Whenever,  in  a  plant  society,  competition  between  individuals 
of  the  same  or  different  species  becomes  a  factor,  the  moisture  prob- 
lem is  different  from  that  which  confronts  seedlings.  In  the  forest 
there  may  be  competition  for  moisture  without  keen  competition  for 
light,  but  the  two  will  usually  be  closely  interrelated.  In  any  ordi- 
nary situation  the  keenest  competition  for  moisture  occurs  near  the 
end  of  the  growing  season,  when  the  reserve  winter  moisture  has 
been  exhausted  and  the  current  rate  of  use  is  in  excess  of  the  current 
accretion.  Where  this  is  the  case  the  study  of  soil  moisture  may  be 
restricted  to  a  period  of  two  or  three  months  in  the  late  part  of  the 
season. 

It  is  evident  that,  of  two  individuals  on  the  same  site,  one  may 
possess  an  advantage  over  the  other  through  deeper  rooting.  It  is 
therefore  essential  in  each  site  studied  to  know  the  extent  and  depth 
of  the  roots  of  the  plants  under  observation,  and  to  sample  the  soil 
for  moisture  in  accordance  with  a  root  map. 

If  it  should  appear  that  two  individuals  in  competition  have  essen- 
tially the  same  moisture  supply,  then  it  obviously  becomes  necessary 
to  determine  their  respective  relations  to  that  supply  by  examination 
of  their  internal  conditions  as  affected  by  atmospheric  conditions, 
light,  etc.  It  is  not  sufficient  in  these  circumstances  to  say  that  soil 
moisture  is  not  a  factor  in  the  greater  success  of  one  than  of  the 
other.  By  measurmg  the  osmotic  pressures  of  the  plants  it  may  be 
found,  for  example,  that  the  individual  which  is  most  exposed  to 
light,  wind,  and  other  desiccating  influences  has  a  greater  control 
over  soil  moisture  than  the  near-by  individual  which  is  shaded  and 


142         BULLETIN  1059,   U.   S.  DEPARTMENT  OF  AGRICULTURE. 

protected,  and  which,  nevertheless,  may  die  from  the  effects  of 
drought  while  the  other  thrives.  Unless,  then,  it  is  desired  to  deal 
in  generalities,  and  ascribe  to  one  species  certain  characters  which 
another  does  not  possess,  on  the  basis  of  general  knowledge  of  pure 
3umption,  it  will  be  necessary  in  studying  competition  to  go  to  the 
whole  length  prescribed  for  the  determination  of  the  coefficient  of 

availability. 

hi.  Much  the  same  situation  exists  with  respect  to  the  study  of  the 
final  character  of  the  forest  formation,  its  height,  rate  of  growth, 
etc.  The  critical  study  of  the  moisture  conditions  in  relation  to 
ultimate  height  is  of  special  importance  in  forestry  because  ultimate 
height,  in  forest  management  plans,  is  often  taken  as  a  criterion  of 
the  quality  of  the  site;  that  is,  its  yield  possibilities.  Are  the  two 
things  closely  related?  If  the  ultimate  height  is  limited  by  drought 
conditions  which  occur  only  periodically,  may  not  the  yield  possi- 
bilities be  considerably  greater  or  less  than  would  be  indicated  by 
this  measure,  depending  very  much  on  the  total  water-holding 
capacity  of  the  soil,  etc.  ?  Obviously  such  questions  as  these  can  be 
answered  only  after  exhaustive  study.  This  requires  the  establish- 
ment of  permanent  soil-moisture  stations,  the  determination  each 
year  of  the  critical  conditions  that  exist,  and  the  use  of  the  entire 
system  that  has  been  suggested  for  arriving  at  a  measure  of  the  true 
conditions. 

Special  Equipment  for  Soil  Moisture  and  Soil  Quality  Study. 

il  sieves: 

Standard  soil  sieves.     Per  set,  about $10.  00 

1  borers: 

3-foot  soil  borer,  tube,  with  hammer 9.  00 

fi-foot  soil  borer,  tube,  with  hammer 11.  00 

6-foot  soil  borer,  auger  handle  hammer 12.  00 

Soil  cans: 

Patent  seamless,  noncorrosive,  tin  soil  cans — 

4-ounce  can,  deep  style,  per  gross 3.  50 

8-ounce  can,  deep  style,  per  gross 4.  00 

(35  per  cent  discount  on  above  prices  to  United  States  Govern- 
ment.) 
Soil  sample  cans,  seamless  tin,  No.  9178  A — 

Capacity,  ounces 4  g  i  g 

Perdozen $0.  22        $0.33  $0.55 

Soil  sample  cans,  aluminum,  with  screw  tops — 

2\  inches  in  diameter  by  2\  inches  high,  unnumbered  9 183, each 25 

With  can  cover  numbered.     (In  ordering,  state  what  numbers  are 

desired.)     No.  9183  A,  each 30 

Soil  sample  can,  aluminum,  with   aluminum   top.     The   diameter   of 
these  cans  is  uniform,   so  that  the   cover   fits   the    bottom   of    the 


RESEARCH  METHODS  IN  STUDY  OF  FOREST  ENVIRONMENT.      143 

Soil  cans — Continued. 

can,  making  it  possible  to  keep  can  and  cover  together  while  the 
can  is  open — 

Number 1  2  3 

Diameter,  inches 2  2\  3£ 

Height,  inches f  If  2 

Each $0.  11         $0.  15        $0.  20 

With  can  and  cover  numbered.     (In  ordering,  state  what  numbers 
are  desired.) 

No.  9184  A,  each $0.  16      $0.  20        $0.  25  . 

Aluminum  soil  cans,  2\  by  2\  inches,  with  screw  tops — 

Lightweight,  per  hundred $38.  00 

Heavyweight,  per  hundred 45.  00 

Cans,  galvanized,  4  by  h\  inches,  for  capillarity,  moisture  equivalent, 

etc.     (Any  sheet-metal  works.) .40 

Drying  ovens: 

About 
Hot-water  bath,  in  various  dimensions  from  9  inches  up -      $50.  00 

and  up. 
Electric,  Freas,  type  R  No.  108,  inside  dimensions  16  by  14  by  16  inches, 

thermostat,  thermometer,  etc $210.  00 

Hearson  low  temperature  incubators,  gas  and  electric  heated,  various 

sizes 140  to  360.  00 

Potentiometers  and  other  electrical  resistance  apparatus. 
Water-retention  cup,  for  determining  the  maximum  water  retained  by  soil,  of 
brass  2  inches  in  diameter  by  f  inch  high,  with  diaphragm  of  perforated 

metal  fastened  about  ^  inch  below  top,  No.  9295 0.  20 

Capillary  moisture  pans: 

Hilgard's  small  circular  metal  pans,  about  1  centimeter  high  and  4£ 
inches  in  diameter,  with  perforated  bottoms  for  determining  '  'capillary 

moisture' '  of  soil,  each 1.  25 

Per  ten 10.  00 

Balances,  glassware,  reagents,  etc.     (Obtainable  from  all  dealers  in  laboratory 
supplies.) 

ATMOSPHERIC  HUMIDITY. 

The  humidity  of  the  atmosphere  is  directly  reflected  in  any  such 
water-containing  object  as  the  leaf  of  a  plant,  in  which  there  is  a 
constant  tendency  to  come  into  vapor-pressure  equilibrium  with  the 
atmosphere,  usually  through  evaporation  but  in  rare  circumstances 
through  absorption.  The  point  of  equilibrium  between  the  leaf  and 
the  atmosphere  will  be  better  understood  by  considering  the  discus- 
sion which  has  preceded,  in  reference  to  osmotic  pressures. 

Although  this  constant  tendency  is  nearly  always  causing  the  loss 
of  water  from  plants,  the  humidity  of  the  atmosphere  alone  can  not 
be  taken  as  a  measure  of  the  " evaporation  stress,"  or  rate  of  evapo- 
ration, depending  on  the  wind  movement  which  aids  in  diffusion  of 
vapor,  and  the  heat  supply,  principally  from  sunlight.  For  this 
reason,  when  a  direct  measure  of  the  evaporation  stress  is  possible 
through  the  use  of  some  form  of  atmometer,  ecological  studies  will 
not  require  the  measurement  of  atmospheric  humidity  except  in  a 


/~i  ft  *-  1  »~*  <->*  r\.  -m  *»  ^~v  w%e~% 


144  BULLETIN   1059,   U.   S.   DEPARTMENT   OF   AGRICULTURE. 

evaporation  records,  or  possibly  for  comparing  conditions  locally 
studied  with  stations  for  which  there  is  no  evaporation  record,  but 
which  maintain  a  complete  record  of  wind  movement,  sunshine 
duration,  and  humidity.  In  such  an  event  it  may  be  possible  to 
work  out  a  fairly  constant  relation  between  the  evaporation  from 
any  given  type  of  atmometer,  and  a  combination  of  these  other  con- 
ditions, properly  integrated,  in  the  general  relation  of: 

/•     A)  =  (Wind  movement  plus  saturation  deficit)  sunshine. 

The  term  "vapor  pressure"  expresses  the  weight  per  cubic  foot, 
or  the  pressure,  in  centimeters  of  mercury,  of  the  vapor  currently 
in  the  atmosphere.  The  term  " saturation  deficit"  expresses  the 
lack  of  vapor  pressure,  or  the  difference  between  the  existing  vapor 
pressure  and  that  which  the  atmosphere  would  contain  at  the  current 
t  cinperature  if  the  space  were  saturated  with  water  vapor.  The  "dew 
point"  indicates  the  temperature  at  which  the  existing  vapor  would 
condense;  or,  in  other  words,  the  temperature  at  which  the  existing 
vapor  would  produce  a  condition  of  saturation.  It  is  readily  seen, 
then,  that  the  saturation  deficit  is  the  difference  between  saturation 
pressure  for  the  current  dry-bulb  temperature,  and  saturation  pres- 
sure for  the  temperature  of  the  current  dew  point.  The  term  "rela- 
tive humidity"  expresses,  as  a  percentage,  the  relation  between  the 
existing  vapor  and  that  which  might  be  present  if  the  space  were 
saturated  at  the  current  air  temperature. 

The  dew-point  figure  is  used  only  incidentally  in  computing  vapor 
pressure,  saturation  deficit,  or  relative  humidity.  Of  the  three, 
experience  in  a  number  of  forest  ecological  studies  has  shown  that 
the  saturation  deficit  is  by  far  the  most  useful,  giving,  as  it  does 
without  further  reference  to  temperature  conditions,  a  direct  measure 
of  the  capacity  of  the  atmosphere  for  more  vapor,  and  hence,  in 
some  degree,  a  measure  of  the  rate  at  which  evaporation  will  take 
place. 

The  psychrometer,  consisting  of  a  pair  of  thermometers  mounted 
on  a  frame  in  such  manner  as  to  be  readily  whirled  in  order  to 
accelerate  evaporation,  is  the  common  instrument  for  determining 
atmospheric  humidity.  One  of  the  thermometers  is  covered  with  a 
layer  of  cloth  (preferably  linen),  which  is  dipped  in  clean  water 
before  making  the  exposure.  The  evaporation  of  this  water  cools  the 
thermometer,  or,  as  the  expression  is,  causes  "a  depression  of  the 
wet  bulb";  and  the  maximum  depression  which  it  is  possible  to 
produce  by  vigorous  movement  of  the  instrument  through  the  air. 
taken  with  the  current  temperature,  is  considered  to  give  a  measure 
of  the  atmospheric  humidity.  Tables  have  been  worked  out.  after 
experiment,  for  almost  all  possible  combinations  of  air  temperatui 
and  wet-bulb  depressions,  showing  the  corresponding  dew  points 
and  relative  humidities.     Of  these  the  best-known  in'  this  country 


RESEARCH  METHODS  IN  STUDY  OF  FOREST  ENVIRONMENT.       145 

are  the  " Psvehrometric  Tables'1  of  the  United  States  Weather 
Bureau,  contained  in  its  Bulletin  235.  These  have  been  worked  out 
for  barometric  pressures  of  30,  29,  27,  and  23  inches.  In  accordance 
with  American  custom,  vapor  pressures  are  given  in  inches  of 
mercury.  Through  the  courtesy  of  the  Weather  Bureau,  it  is  possi- 
ble to  produce  in  the  Appendix  an  additional  table  of  vapor  pres- 
sures for  a  mean  barometer  of  21.42  inches,  prepared  by  B.  C.  Kadel 
for  the  special  use  of  the  Wagon  Wheel  Gap  Experiment  Station, 
at  an  elevation  of  9,300  feet.  This  table  will  doubtless  be  of  con- 
siderable assistance  in  ecological  studies  in  the  western  mountains. 

The  vapor  pressure  may  also  be  determined  very  quickly  and 
precisely  by  means  of  dew-point  apparatus  and  a  table  of  satura- 
tion pressures  corresponding  to  various  temperatures.  This  appa- 
ratus is,  however,  far  less  convenient  for  field  use  than  the 
psychrometer. 

The  ideal  record  of  humidity  is,  of  course,  one  which  shows  the 
atmospheric  condition  for  every  hour  of  the  day.  Theoretically,  this 
is  obtained  by  the  use  of  the  hair  hygrograph;  but,  actually,  the 
instrument  is  of  very  little  use. 

The  atmospheric  conditions  are  measured  in  terms  of  relative 
humidity,  which  fluctuates  rapidly  with  every  change  in  air  tem- 
perature. The  record  must  then  be  transposed,  in  conjunction  with 
the  continuous  temperature  record,  into  terms  of  absolute  humidity 
and  saturation  deficit,  before  it  can  have  much  value.  Furthermore, 
the  hygrograph  is  probably  the  least  reliable  and  accurate  of  the 
automatic  instruments  commonly  used. 

Since  the  absolute  humidity  or  vapor  pressure  usually  does  not 
change  through  a  wide  range  in  a  short  time,  but  shows  a  general 
tendency  to  increase  as  the  air  warms  and  to  decrease  with  the 
cooling  at  night,  it  is  possible  to  determine  a  fairly  satisfactory  mean 
humidity  for  any  day  (except  of  course  during  general  disturbances) 
by  means  of  two  or  three  observations  with  the  psychrometer.  For 
example,  the  hours  of  7  a.  m.,  1  and  7  p.  m.,  have  been  used,  or  7  a.  m., 
2  and  9  p.  m.  After  hourly  observations  for  a  few  days  at  any 
season  and  point,  it  should  be  possible  to  select  one  or  more  con- 
venient hours  when,  in  the  ordinary  sequence  of  events,  the  mean 
humidity  of  the  day  may  be  approximately  measured,  either  at  each 
observation,  or  through  averaging  unlike  valuations.  As  has  been 
suggested,  the  absolute  humidity  varies  less  than  the  relative  humid- 
ity or  saturation  deficit.  Therefore,  for  calculating  the  mean  sat- 
uration deficit  for  the  day  it  is  logical  to  arrive  first  at  the  mean 
vapor  pressure,  and  then,  after  calculating  the  mean  temperature 
for  the  whole  period,  to  obtain  the  saturation  deficit  by  deduction. 
10163— 22— Bull.  1059 10 


146  BULLETIN   1059,   U.   S.   DEPARTMENT   OF   AGRICULTURE. 

If  only  one  psychrometer  reading  a  day  is  feasible,  both  the  wet 
and  the  dry  bulb  reading  may  be  entered  in  the  first  columns  of  the 
"Humidity,  Wind,  and  Evaporation"  form,  and  the  relative  humid- 
it  v,  vapor  pressure,  and  saturation  deficit  calculated  therefrom  may 
I  opposite.  When  several  readings  are  made  each  day,  it  is 
suggested  that  the  calculated  vapor  pressure  be  recorded  on  the 
form  for  " Hourly  (Air,  Soil,  or  Actinograph)  Temperatures"  for 
their  appropriate  hours,  and  that  only  the  mean  vapor  pressure  be 
recorded  on  the  "Humidity,  Wind,  and  Evaporation"  form. 

The   relative   humidity,    vapor   pressure,    and   saturation    deficit 

should  be  averaged  by  decades  and  months.     The  means  by  months, 

the  year,  and  the  growing  season,  should  be  shown  on  the  annual 

"Summary"  form. 

Instruments. 

Psychrometer,  sling,  standard  Weather  Bureau  pattern;  aluminum  backs: 
polished  hardwood  handles;  double-length  connections;  2  glass  tubes, 
exposed,  mercurial  thermometers,  9  inches  long;  stem-graduated  and 
figure  on  glass  for  each  10  degrees;  Fahrenheit  or  centigrade $3.  00-|6. 00 

Whirling  apparatus,  stationary,  complete  (without  thermometers) 18. 50 

Cog  psychrometer,  Thermometers  about  4h  inches  long,  reading  —  5°  to 
50°  C,  No.  1230 4.50 

Hydrograph  (or  self-registering  hygrometer)  complete  with  a  year's  sup- 
ply blank  forms.     No.  58-B,  pen  and  ink 80. 00 

WIND   MOVEMENT. 

Wind  movement  may  be  effective  upon  plants  both  directly  and 
indirectly;  that  is,  through  mechanical  breakage,  windfall,  etc.,  and 
through  its  influence  upon  evaporation  and  transpiration. 

While  mechanical  injury  to  trees  by  wind  seems  to  be  a  less  im- 
portant factor  in  American  forests  than  in  those  of  Europe,  judged 
by  the  literature  on  the  subject,  the  problem  of  windfalls  is  one  of 
ever-growing  importance  as  forestry  is  extended  and  thought  is  given 
to  the  conservation  of  that  portion  of  the  stand  which  is  not  now 
merchantable  or  is  needed  as  a  guarantee  of  future  reproduction.  A 
recent  article  by  Weidman  (150)  and  several  other  articles  that 
might  be  cited  have  shown  the  importance  of  the  problem  and  the 
desirability  of  a  great  many  more  wind  records  than  are  now  avail- 
able for  our  forest  regions,  if  the  problem  is  to  be  scientifically  solved. 
Perhaps  this  is  a  far  cry  from  ecology.  Yet  a  disturbance  in  the 
forest  which  is  capable  of  starting  a  new  succession  is  certainly  of 
some  ecological  significance,  at  least  after  it  has  occurred. 

Wind  movement  has  without  doubt  a  very  marked  effect  on  evapo- 
ration; and,  in  addition,  the  moving  air  may  be  either  a  source  of 
heat  or  a  means  of  dissipating  the  heat  of  sunlight,  as  suggested  by 


RESEARCH  METHODS  IN  STUDY  OF  FOREST  ENVIRONMENT.      147 

Bates  (145)  in  discussing  the  actual  measure  of  heat  available  for 
use  in  the  plant.  The  first  of  these  influences  may  be  practically 
ignored,  as  was  the  case  with  humidity,  if  there  is  available  a  record 
which  has  integrated  all  the  factors  in  evaporation.  It  is  believed, 
however,  that  most  ecological  studies  will  be  found  deficient  if  the 
record  of  wind  movement  is  not  obtained. 

To  obtain  a  record  of  wind  movement  in  the  forest  which  may  cause 
mechanical  injury,  the  anemometer  should  undoubtedly  be  placed 
almost  at  the  tops  of  the  tree  crowns,  where  the  most  severe  winds 
will  be  encountered.  A  strong  support  is  needed  to  prevent  loss  of 
record  at  the  most  critical  times. 

In  the  study  of  reproduction  and  of  other  shall  plants,  it  may  even 
be  necessary  to  dig  a  pit  for  the  stem  of  the  anemometer  in  order  that 
the  cups  may  be  close  to  the  ground  surface. 

The  standard  Robinson  anemometer  is  the  most  practical  instru- 
ment for  all  outside  work.  Because  of  a  friction  factor,  it  underrates 
wind  of  low  velocity  such  as  is  often  characteristic  of  the  forest  floor, 
and  slightly  overrates  the  high  velocities.  The  amount  of  wind  move- 
ment may  be  read  on  the  dial  of  the  instrument  to  tenths  of  miles, 
and  the  anemometer  may  also  be  electrically  comiected  to  a  register 
so  as  to  give  a  record  of  each  mile  of  wind  movement.  Because  it 
records  no  less  than  a  mile  of  wind  movement,  the  Robinson  ane- 
mometer is  not  wholly  satisfactory  from  the  standpoint  of  mechani- 
cal injury  to  trees.  It  is  possibly  more  true  in  mountainous  regions 
than  elsewhere  that  the  winds  of  greatest  velocity  are  gusty,  and  it 
seems  likely  that  the  gusts  of  only  a  few  seconds'  duration  may  have 
at  least  twice  the  mean  velocity  recorded  for  whole  miles.  While 
daily  or  more  frequent  readings  of  the  anemometer  dial  may  be 
sufficient  where  a  definite  use  of  the  wind  record  can  not  be  foreseen, 
in  many  cases  the  occurrence  of  maximum  and  minimum  velocities, 
the  movement  by  day  and  by  night,  etc.,  as  obtainable  from  the  elec- 
trically operated  register,  will  be  desired.  Since  the  current  required 
for  operation  is  only  for  2  or  3  volts,  connection  with  the  anemometer 
in  the  field  may  be  made  with  the  crudest  sort  of  conductors,  using 
wire  fences,  or  insulated  wire  laid  on  the  ground.  In  this  way  the 
register  may  be  in  a  protected  place  and  receive  due  attention. 

Apparently  the  only  apparatus  capable  of  recording  momentary 
high  velocities  is  the  Dines  pressure-tube  anemometer,  the  use  of 
which  in  the  forest  is  hardly  feasible. 

Wind  vanes  with  connections  and  registering  device  are  obtain- 
able, and  may  possibly  be  desired  at  one  station  in  a  locality.  There 
is,  however,  no  ecological  significance  in  wind  direction;  and  if  there 
were,  it  is  probable  that  a  single  observation  on  prevailing  direction 
each  day  and  night  would  be  amply  sufficient. 


148         BULLETIN   1059,   U.   S.   DEPARTMENT   OE    AGRICULTURE. 


_ 


CO 


- 


Z 

O 
i— i 
H 
< 

O 

- 

> 
- 


-  Q 

.a  < 

I  £ 

-- 

-  — 
;-  > 

■  E- 

rf.  — ■ 


c 


E 

z 

— 


- 


s 


=0 

v. 

C 


o 


o 


05 


£ 
o 

'A 


I 


S 


c 


Si 

0 


> 


Re- 
filled 
to. 

0 

sa 

93 

7. 

Water 

loss. 

Cm. 

Eh 

oa 

^ 

Read. 

03 

TZ           1 

Re- 
filled 
to. 

Cm. 



- 

oa 

Water 

loss. 

S 
c 

r- 

Eh 

Mead 

Cm. 

03 

^ 

From  hourly  record.2 

Nighl 
Mov't. 

Mi. 

; 

Day 

Mov't. 

Mi. 

Max. 
Mov't 
1  Hr. 

s 

Wind  movement. 

Prevailing  direction. 

24 

Hours. 

Dir. 

— 

2 

s 

- 

Dir. 

— 
•-— 

o 
> 

Move- 
ment.1 

Mi. 

Dial. 

Mi. 

oa 

- 

KM 

o 

EH 

W 

• 

Sat.  def. 

Ins. 

>. 

Vapor 

press. 

Ins. 

1 

Rel. 

hum. 

^ 

oj  ~ 

0 

33 

- 

Dry 
bulb. 

0 

- 

03 

03 

— 
oa 

- 

» 

- 

!■* 

■  - 

t- 

<" 

3 

r 

d 

B 

— 

- 

■• 

— 

.  - 

z 

t- 

« 

- 

q 

— 

— 
— 

RESEARCH   METHODS  IX  STUDY  OF  FOREST  ENVIRONMENT.      149 


03   o! 

03 

EH 

03  03  03 

o3  o3  o3 







H 



off  A  flfl 

c3  03  c3 

•    ••••••••.•■ 

!    ;    ; 

~  s  ~ 

03  03  03 

C'l  o»  ci  r«  c-i  c)  t>  o  ?i  ?s  ?o 

Dec.  mean.  . 
Sum. month. 
Mean  month. 

1- 
© 


,.  S 

si 

—    CO 

•=  2 

i.  — 


—   era 

a  fe 

W  - 
© 

w       C 

^  w  c 
■H  ®  S3 

£  ©  03 

«  w  o 

~  .7  x 

•C  2  03 
©;;  © 

©  C  ~ 

©  i— I 

§  ••  .2 
v.  &JC 

'Oflfc 
s  —  o; 
*    r« 

©    ■  g  e 

—  —  3  ■— 
O 


S.co  S 

"*  o  ©  5? 

Jq    •'5  u 

+*  d        03 
IS     .-2  03 

fs*-  c 

111  08 

©  fe  5 
2  P<w  ,• 

g  S*^ 

©  >  »•-  t, 

e  o     « 

a  °  s  £ 

^    Q    O    S 

©  *^'S  *z 
.fl+a  03  C 

«  c  >  a 
&  2  ©_r 

-  S  s  * 

-  **  °-S 

si'3?; 

do  r  2-- 

-C  03.5^ 

—«     C*     W     ^ 


150         BULLETIN   1059,   U.    S.   DEPARTMENT   OF   AGRICULTURE. 

For  all  stations  the  dial  reading  should  be  recorded  in  the  field  at 
each  observation,  and  the  dial  readings  and  total  movements  should 
be  tabulated  in  adjacent  columns  of  the _" Humidity.  Wind,  and 
Evaporation"  form.  The  decade  means  or  sums  and  the  monthly 
sums  and  means  should  be  computed  and  entered.  For  most  uses 
the  decade  sums  will  be  preferable  to  means.  The  record  of  day  and 
nighl  movements  may  be  obtained  either  from  two  dial  readings 
per  day,  or  from  hourly  automatic  records. 

Where  hourly  records  are  available,  the  greatest  wind  movement 
for  any  single  hour  of  each  day  should  be  entered,  and  also  the  total 
wind  movement  by  day  and  by  night;  that  is,  from  6  a.  m.  to  6  p.  m. 
and  from  6  p.  m.  to  6  a.  m.  This  division  of  the  daily  movement  is 
of  interest  in  the  general  study  of  the  climate,  and  may  assist,  for 
example,  in  determining  relative  amounts  of  evaporation  during  the 
day  and  night,  when  this  is  not  readily  determined  directly. 

The  monthly,  annual,  and  growing  season  means  of  the  movements 
obtained  from  dial  readings,  and  of  maximum  hourly  velocities,  and 
the  sums  of  clay  and  night  movements  obtained  from  automatic 
records,  with  the  mean  hourly  movements  computed  therefrom,  may 
be  tabulated  on  the  "  Summary  "  form. 

Instruments. 

Self-registering  instruments: 

Single  register  complete  (for  wind  velocity),  with  a  year's  supply  blank 
Forms  1015,  pen  and  ink  (by  special  arrangement  a  register  to  carry 

wind  record  for  one  week  may  be  obtained) $95.  00 

Two  magnet  registers — 

No.  2,  for  wind  velocity  and  sunshine  (using  Form  No.  1015-C  ....     125.  00 

No.  3,  for  wind  velocity  and  rainfall  (using  Form  No.  1015-D 1 130.  00 

No.  4,  for  wind  velocity,  rainfall,  and  sunshine  (using  Form  No. 

1015-E) 140. 1  »0 

Quadruple  register  complete  (for  wind  direction,  wind  velocity,  rain- 
fall, and  sunshine),  with  a  year's  supply  of  blank  Forms  1017,  pei 

and  ink 275.  00 

In  all  of  the  above,  cable  and  battery  for  electrical  installation  a 
extra. 
Anemometers,  wind  vanes,  and  supports: 

Anemometer,  Robinson's;  of  the  standard  pattern,  complete  with  alumi- 
num cups  and  wrench;  all  dimensions  and  construction  to  be  identical 
and  small  parts  interchangeable  with  corresponding  parts  of  the  ane- 
mometer of  the  Weather  Bureau 00 

Anemometer  cups,  aluminum,  with  arms,  G.  S.  S.  No.  L2200;  per  Bel ...         -  00 
Anemometer  cups,  copper,  reinforced,  .013  inch  thick,  hemmed  al 

and  arms  reinforced,  G.  S.  S.  No.  12201;  per  set 9.  50 

Wind  vane;  small,  for  towers,  with  mounting,  complete.  «-.  S.  S.  No 
122?4 13.50 


RESEARCH  METHODS  IN  STUDY  OF  FOREST  ENVIRONMENT.      151 

Anemometers,  wind  vanes,  and  supports — Continued. 

Combined  wind  vane  and  anemometer  support,  20  feet  high,  adapted  for 
use  with  quadruple  register;  complete  with  6-foot  vane,  electrical 
contacts,  etc.,  but  without  anemometer f  105.  50 

Wind  vane,  4  feet,  on  7-foot  support,  with  direction  arms,  gilt  letters, 

and  anemometer  support,  but  without  anemometer 35.  00 

"Wind  vane  and  support,  as  above,  but  without  anemometer  or  anemom- 
eter support  arm 27.  50 

Support  for  anemometer  alone,  without  vane,  direction  arms,  or  anemom- 
eter        15. 00 

EVAPORATION. 

Xo  ecological  study  can  be  considered  comprehensive  which  does 
not  take  into  account  the  desiccating  power  or  " evaporation  stress" 
of  the  atmosphere.  While  the  water  supply  of  the  soil  has  been  con- 
sidered as  the  condition  most  directly  determining  the  character  of 
vegetation  on  a  given  site,  and  its  rate  of  growth,  almost  equal 
attention  must  be  given  to  the  matter  of  the  dissemination  through 
transpiration  of  the  moisture  winch  reaches  the  plant.  The  evapora- 
tion rate  in  different  habitats  will  perhaps  be  found  to  show  greater 
variation  than  any  other  condition.  It  is  especially  valuable  when 
measured  directly,  because  it  gives  the  integrated  effect  of  wind, 
humidity,  air  temperature  and  sunshine — an  integration  which  can 
not  be  accomplished  by  any  artificial  means.  While  it  is  not  to  be 
expected  that  any  instrument  will  integrate  the  effect  of  these  differ- 
ent stimuli  to  evaporation  in  a  manner  corresponding  to  their  com- 
bined effect  on  the  plant,  yet  this  is  the  object  to  which  the  greatest 
efforts  have  been  bent  and  which  has  to  some  extent  been  attained. 

The  study  of  the  evaporation  factor  may  be  made  directly,  of 
course,  by  observing  the  transpiration  of  plants  in  the  field.  While 
very  desirable  and  not  very  difficult,  this  is  perhaps  less  satisfactory 
than  the  instrumental  method  in  reducing  the  conditions  of  the  en- 
vironment to  physical  terms.  Since  this  discussion  is  mainly  con- 
cerned with  instrumental  procedure  in  forest  investigations,  the 
instrumental  method,  even  though  less  desirable  than  direct  measure- 
ments of  transpiration,  will  be  considered  first. 

Objects  and  Nature  of  Evaporation  Measurements. 

There  may  be  two  rather  distinct  objects  in  measuring  evaporation 
rates,  although  in  ecology  there  is  only  one.  Climatologists,  irriga- 
tion engineers,  etc.,  may  desire  to  know,  for  local  conditions  and  for 
general  comparative  purposes,  how  much  capacity  the  atmosphere 
possesses  day  by  day  and  year  by  year  to  take  up  moisture  when 
offered  moisture,  as  freely  as  possible,  from  the  surface  of  a  body 
of  water.  Obviously  this  does  not  measure  the  capacity  of  the 
atmosphere,  which  could  better  be  determined  by  humidity  observa- 


152  BULLETIN   1059.    TJ.    S.    DEPARTMENT    <>E    AGRICULTURE. 

tions.  For  crude  comparative  purposes,  however,  the  loss  from  a  pan 
of  water  is  probably  as  good  a  measure  as  any  of  general,  regional, 
atmospheric  conditions,  such  as  would  affect  lakes  and  reservoirs, 
provided  only  a  standard  exposure  is  employed. 

In  plant  ecology  evaporation  rates  are  measured  for  the  purpose 
of  determining  how  great  or  how  little  evaporation   tendency,   or 

ress,  the  plant  has  to  withstand  and  can  withstand  without  injury. 
As  has  been  pointed  out  by  Bates  (151)  under  any  circumstances 
the  components  of  evaporation  are  (1)  the  heat  necessary  to  trans- 
form water  into  vapor,  and  (2)  the  diffusion  possibilities  of  the 
vapor.  In  ordinary  evaporation  there  are  two  possible  sources  of 
heat:  namely,  sunlight,  or  radiation  from  any  other  source,  and  the 
heat  of  the  air  coming  in  contact  with  the  evaporating  body.  By 
diffusion  possibilities  are  meant  the  net  opportunities  for  the  vapor 
bo  move  away  from  the  evaporating  body.  These  depend  on  me- 
chanical obstructions,  on  the  number  of  vapor  molecules  already  in 
the  atmosphere,  on  the  air  movement  near  the  evaporating  body  affect- 
ing this  number,  but  most  of  all  on  the  temperature,  energy,  and 
pressure  of  the  molecules  which  are  moving  away.  Hence  sunlight 
and  almost  every  atmospheric  condition  affect  evaporation,  and  it  i- 
also  plainly  evident  that  each  of  these  factors  can  not  have  an  identi- 
cal effect  on  each  of  several  different  evaporating  bodies.  To  use 
an  extreme  example,  a  highly  polished  metal  vessel  containing  water 
might  be  set  in  full  sunlight,  and  yet  the  evaporation  of  the  water 
would  not  be  appreciably  influenced  by  the  sunlight,  because  the  heat 
of  the  sun  would  not  reach  it — the  rays  would  be  almost  totally 
reflected.  Similarly,  the  evaporating  body  may  be  very  largely  pro- 
tected from  wind,  so  far  as  the  wind  might  affect  diffusion.  This 
i-  to  some  extent  the  case  in  the  leaf,  where  the  vapor  is  largely 
formed  internally. 

When,  therefore,  it  is  said  that  in  ecology  an  atmometer  is  desired 
which  will  react  to  the  same  external  stimuli  as  affect  tin-  plant  leaf 
most  directly,  it  is  not  by  any  means  implied  that  the  instrument 
would  parallel  the  plants'  transpiration  under  all  circumstance-. 
There  should  be  a  certain  similarity  in  their  reactions  when  both  are 
reacting  freely.  It  is  not  desirable  to  compare  an  instrument  which 
is  mainly  susceptible  to  wind  with  a  plant  which  responds  in  the 
largest  measure  to  sunlight.  If  there  could  be  an  atmometer  which 
would  receive  its  stimulus  from  sunlight,  wind,  air  temperature,  and 
humidity  in  about  the  same  proportions  as  these  four  factors  affect 
the  plant,  the  evaporation  factor  of  the  habitat  could  be  measured 
in  an  effective  way.  Otherwise,  altogether  too  much  emphasis  may 
be  placed  on  some  one  component,  such  as  wind.  After  all  is  said 
almost  exactly  the  same  problem   as  attempting  to  integrate 


RESEARCH  METHODS  IN  STUDY  OF  FOREST  ENVIRONMENT.       153 

mathematically  the  effects  of  the  several  components  on  the  evapora- 
tion from  the  plant.  One  is  just  as  far  from  an  absolutely  precise 
outcome  as  the  other.  The  advantage  of  the  atmometer  is  that,  once 
the  integration  has  been  accomplished  in  the  construction  of  the  in- 
strument, the  observations  are  relatively  simple  and  no  further  com- 
plex calculations  are  necessary. 

As  has  been  suggested,  it  is  not  to  be  presumed  that  the  atmometer 
will  show  restricted  evaporation  due  to  control,  as  may  be  the  case 
with  the  plant  when  it  closes  its  stomata  or  when  transpiration  is 
automatically  reduced  by  increasing  density  of  the  cell  sap.  By 
comparing  the  most  perfect  atmometer  with  plants,  however,  it 
should  be  possible  to  measure  the  actual  effectiveness  of  these  plant 
controls. 

It  may  be  worth  while  to  suggest,  for  the  sake  of  more  effective 
evaporation  studies,  that  it  is  possibly  erroneous  for  the  student 
of  plant  life  to  look  upon  a  large  evaporation  factor  in  the  habitat 
as  necessarily  inimical  to  the  plants  which  ape  there.  Some  theo- 
retical considerations  which  point  to  transpiration  as  a  benefit  have 
already  been  outlined.  No  attempt  will  be  made,  however,  to  decide 
the  question  as  to  whether  it  is  beneficial  to  the  plant  or  merely  a 
necessary  evil.  Laying  this  question  aside,  it  is  perfectly  evident 
that  conditions  conducive  to  high  transpiration  rates  are  an  unavoid- 
able concomitant  of  the  conditions  necessary  to  active  photo-synthesis. 

When  therefore,  as  in  Weaver's  (169)  succession  from  prairie  to 
brush  or  woodland  types,  it  is  found  that  succession  produces  a  stead- 
ily decreasing  evaporation  rate,  shall  it  be  concluded  that  the  plants 
of  the  brush  stage  are  directly  favored  by  the  decreased  evapora- 
tion, only  relatively  favored,  or  not  helped  at  all,  but  merely  able  to 
succeed  with  less  sunlight  than  the  plants  of  the  prairie?  Very 
likely,  in  a  case  of  this  kind,  the  rate  of  evaporation,  while  a  service- 
able index  to  the  general  conditions,  may  not  itself  be  a  controlling 
factor,  or  may  be  a  controlling  factor  only  for  a  brief  period  in  each 
season  when  drought  occurs.  It  would  appear  to  be  all-important 
that  evaporation  rates  be  closely  correlated  with  the  moisture  of  the 
soil,  as  is  done  by  Shreve  (166)  in  giving  directly,  if  somewhat 
crudely,  the  ratio  of  evaporation  to  soil  moisture  contents  in  various 
habitats.  What  is  perhaps  more  important  is  that  a  clear  distinc- 
tion should  be  made  between  evaporation  stresses  when  there  is  an 
abundance  of  soil  moisture,  and  those  existing  when  the  moisture 
supply  is  nearly  exhausted.  There  are,  also,  critical  periods  brought 
about  by  excessive  evaporation  when  the  soil  moisture  is  apparently 
all  that  it  should  be.  These  and  their  effects  must  be  separately 
analvzed. 


154  BULLETIN   1059,   U.   S.   DEPARTMENT   OF   AGRICULTURE. 

The  main  object  of  this  discussion  is  to  make  clear  the  need  for 
evaporation  records  day  by  day,  and  for  a  finer  analysis  of  the  sea- 
sonal records  than  has  been  the  custom  in  recent  ecological  studies. 

Instrumental  Methods. 

The  instrumental  methods  of  measuring  evaporation  may  be  of 
two  kinds,  employing  respectively  free-water  bodies,  as  in  the  open 
tank,  and  evaporating  surfaces  which  are  kept  continually  moist 
but  in  which  the  water  is  retained  and  to  some  extent  withheld  from 
evaporation  by  capillary  action.  The  latter,  for  brevity's  sake,  will 
be  spoken  of  as  nonfree-water  methods  and  instruments.  The  litera- 
ture of  instrumental  evaporation  studies  is  very  extensive  and  has 
been  fully  annoted  by  G.  Livingston  (162).  This  discussion  must 
be  confined  to  a  few  of  the  more  recent  efforts. 

FREE- WATER  SURFACE. 

The  free-water  surface  receptacle  is  used  almost  exclusively  in 
broad  climatological  and  irrigation  studies  and  only  to  a  limited 
extent  by  ecologists.  The  Weather  Bureau  has  now  adopted  a  stand- 
ard free-water  surface  evaporating  pan,  10  inches  deep.  48  inches  in 
diameter,  and  constructed  of  22-gauge  galvanized  iron,  which  is  de- 
scribed by  Kadel  (155).  Largely  through  the  work  of  Bigelow  (152), 
it  was  determined  to  be  essential,  that  the  receptacle  have  a  surface 
of  known  area,  that  the  water  exposed  in  the  pan  or  tank  have  a 
known  volume,  that  the  surface  of  the  water  have  a  known  and  as 
nearly  constant  distance  below  the  margin  of  the  vessel  as  possible, 
and  that  the  material  and  shape  of  the  receptacle  be  the  same  in  all 
cases.  All  of  these  conditions  are  principally  effective  on  the  tem- 
perature attained  by  the  water,  and  variation  in  any  one  of  them  is 
apt  to  affect  the  evaporation.  The  size  of  the  vessel  and  the  water 
it  contains,  for  example,  have  much  to  do  with  the  hourly  rate  of 
evaporation.  A  small  vessel  of  a  given  aerial  surface  and  depth  will 
give  a  higher  evaporation  than  will  a  vessel  five  times  as  large,  since 
the  water  in  the  smaller  vessel  will  more  readily  adjust  itself  to  con- 
ditions of  the  surrounding  air  than  the  water  in  the  larger  pan.  The 
temperature  of  the  evaporating  medium,  of  course,  is  of  great  im- 
portance in  determining  the  rate  at  which  vapor  rises  from  a  pure 
liquid.  It  is  now  the  standard  procedure  to  select  a  very  open  site 
for  evaporation  measurements,  avoiding  objects  which  might  shade 
the  pan  or  give  it  reflected  light.  The  pan  is  placed  on  a  low  plat- 
form or  crib,  only  a  few  inches  above  the  ground,  vet  allowing  air 
circulation  all  around  it. 

Measurements. 

The  amount  of  water  evaporated  is  determined  by  the  use  of  a 
hook  gauge,  the  loss  in  any  24-hour  period  hem-  corrected  for  pr< 


RESEARCH  METHODS  IN  STUDY  OF  FOREST  ENVIRONMENT.      155 

cipitation,  in  accordance  with  the  measurement  in  a  standard  rain 
gauge  nearby.  The  evaporating  pan  is  filled  to  a  depth  of  8  inches 
at  the  outset,  and  refilled  to  this  amount  whenever  the  water  has 
receded  an  inch.  The  water  is  occasionally  freshened  by  a  complete 
change. 

NONFREE  WATER  SURFACE. 

There  are  three  instruments  which  have  been  sufficiently  used  in 
this  country  in  recent  years  to  warrant  discussion.  Each  of  these 
three  exemplifies   a   different    technical  idea. 

Pichc  evaporimeter. 

The  Piche  evaporimeter,  as  modified  by  the  Weather  Bureau,  was 
used  considerably  10  years  ago  and  has  been  described  by  Russell 
(164).  It  consists  of  a  graduated  glass  tube  as  a  reservoir  for  the 
water  and  a  filter  paper  held  over  the  open  end  of  this  tube  by 
means  of  a  horizontal  glass  plate,  a  spring,  and  a  pressure  screw.  It 
is  commonly  equipped  with  a  10-centimcter  (4-inch)  glass  plate  and 
a  9-centimeter  filter  paper  under  ordinary  conditions,  or  a  51-centi- 
meter paper  of  the  same  make  when  evaporation  is  likely,  between 
observations,  to  exceed  the  capacity  of  the  tube,  about  40  cubic  centi- 
meters. The  larger  paper  exposes  60.91  square  centimeters  and  the 
smaller  21.06  centimeters.  Therefore,  quantities  evaporated  from  the 
smaller  papers  should  be  multiplied  by  2.891  to  make  them  approxi- 
mately comparable  with  the  others. 

Distilled  water  should  be  used  in  evaporimeters,  both  because  of 
the  effect  of  soluble  substances  and  to  keep  the  instruments  clean  and 
free  acting.  A.nonfreezing  solution  of  25  per  cent  denatured  alcohol 
and  75  per  cent  distilled  water  has  sometimes  been  used  in  cold 
weather;  but  the  value  of  records  obtained  under  such  conditions  is 
questionable,  because  at  times  the  evaporation  is  almost  wholly  from 
the  alcohol,  and  the  ratio  between  alcohol  and  water  or  ice  would, 
of  course,  depend  very  largely  on  the  temperature.  For  this  reason 
the  instrument  can  not  properly  be  considered  for  freezing  weather. 

The  regulation  of  pressure  on  the  glass  plate  is  a  somewhat  com- 
plicating and  bothersome  factor.  In  dry  weather  the  pressure  must 
be  made  light  to  feed  the  paper  sufficiently,  and  in  damp  weather  it 
must  be  quite  firm  to  prevent  overflowing  on  to  the  glass,  if  not  actual 
dripping. 

Evaporimeters  of  this  kind  may  best  be  suspended  on  wires,  hav- 
ing hooks  at  their  lower  ends,  so  that  the  instruments  may  be  readily 
taken  down  for  filling.  In  filling,  a  long  50-cubic  centimeter  pipette 
is  found  most  convenient,  making  it  possible  to  keep  the  outside  of  the 
tube  dry.  Care  should  be  taken  to  have  the  filter  paper  wetted  and 
adhering  closely  to  the  glass  before  the  instrument  is  read  and  left. 


156         BULLETIN    1059,   U.    S.    DEPARTMENT   OF   AGRICULTURE. 

It  is  unnecessary  to  calibrate  these  instruments,  because  of*  the  fre- 
quent changing  of  the  filter  papers  and  the  fact  that  papers  of  one 
grade  may  be  quite  uniform  in  their  capillary  properties.  Beyond 
tin-,  the  evaporation  rate  is  somewhat  controlled  by  the  adjustment 
and  the  degree  to  which  the  paper  is  wetted. 

The  Piche  evaporimeter  is  not  now  considered  so  desirable  an  in- 
strument as  some  of  the  other  types.  Though  it  is  simple  and  fairly 
easy  to  operate  under  most  circumstances,  it  is  fragile  and  hardly 
suited  to  severe  weather  conditions;  the  adjustment  of  the  feeding  for 
changes  in  the  weather  is  always  vexatious  and  sometimes  beyond 
one's  power;  a  correction  for  rainfall  is  out  of  the  question:  and  the 
technical  point  may  be  raised  that  the  moist  surface  is  somewhat  too 
freelv  exposed  to  wind,  while  the  white  filter  paper  absorbs  only  a 
-mall  proportion  of  incident  radiation.  The  conditions  for  evapora- 
tion are  therefore  very  different  from  those  within  the  leaf. 

Porous-cup  atmomeier. 

The  name  "atmometer,"  while  describing  any  instrument  for  the 
measurement  of  evaporation  from  a  moist  surface,  is  usually  asso- 
ciated with  evaporimeters  of  the  porous  cup  type.  A  very  satisfac- 
tory field  instrument  of  this  type  has  been  described  in  several 
papers  by  Livingston  (159),  and  may  be  obtained  on  the  market 
either  with  or  without  standardization.  Only  standardized  instru- 
ments should  be  used  in  comparative  studies.  The  instrument  con- 
sists of  a  closed  cup  with  porous  walls,  into  which  the  water  is  fed 
at  any  desired  pressure  by  regulating  the  beight  of  the  reservoir. 
The  reservoir,  connected  with  the  cup  by  rubber  tubing,  may  be  a 
flask  of  any  size  or  a  graduated  tube.  In  the  former  case,  the 
amount  of  evaporation  in  any  specified  period  may  not  be  deter- 
mined directly,  but  rather  by  measuring  the  amount  of  water  re- 
quired to  fill  the  flask  to  its  original  level.  This  feature  is  incon- 
venient, but  the  use  of  a  large  flask  so  increases  the  possibilities  of 
the  instrument  for  long-period  observations  that  it  is.  in  this  re- 
ject, far  superior  to  the  Piche  evaporimeter. 

The  moisture  of  the  cup  reaches  the  outer  surface  through  the 
porous  walls,  its  rate  of  movement  being  determined  by  the  internal 
pressure  and  also  by  the  difference  in  capillary  tension  caused  by 
the  loss  at  the  outer  ends  of  the  pores.  Presumablv  capillary  move- 
ment is  sufficiently  rapid  to  maintain  the  supply  at  the  outer  sur- 
face at  any  reasonable  rate  of  evaporation.  Yet  there  must  be  a 
limit  to  this  capillary  action  in  both  directions,  and  for  this  rea- 
son  the  movement   must,   under   extreme   conditions,    be   governed 

7  H 


RESEARCH  METHODS  IN  STUDY  OF  FOREST  ENVIRONMENT.      157 

somewhat  according  to  atmospheric  conditions  by  regulating  the 
hydrostatic  pressure.  With  the  outside  walls  of  the  cup  always 
moist  and  yet  not  dripping,  the  rate  of  evaporation  will  of  course 
be  governed  by  atmospheric  conditions.  It  must  not  be  expected, 
however,  that  the  evaporation  from  this  instrument  may  be  com- 
pared under  a  variety  of  conditions  with  that  from  the  Piche  in- 
strument, or  with  that  from  a  free-water  surface.  While  the  ab- 
sorption of  heat  from  radiant  sources  and  conduction  from  the  air 
will  be  practically  the  same  for  the  water  surfaces  in  the  three 
cases  mentioned,  yet  the  further  absorption  beyond  the  first  water 
surface  will  depend  on  the  nature  of  the  substance  behind  that 
water  surface— in  the  one  case,  water;  in  the  second,  paper  and  glass; 
in  the  third,  clay  or  some  similar  earthy  substance.  Therefore,, 
the  three  instruments  will  respond  quite  differently  to  the  stimuli 
of  warm  air  and  sunshine. 

For  these  reasons,  comparative  data  will  be  of  value  only  when 
the  same  instrument  is  used  in  all  measurements  of  the  comparison. 

Skive's  nonabsorbent  porous-cup  atmometer. 

It  has  been  the  experience  of  various  investigators  that  the  Liv- 
ingston porous  cup  atmometer  measures  the  evaporating  powTer  of  the 
air  with  a  very  considerable  degree  of  accuracy  during  periods 
when  the  temperature  is  not  recorded   at  or  below  zero  centigrade. 

In  1910,  Livingston  (158)  described  a  rain-correcting  atmometer. 
This  atmometer,  while  giving  great  satisfaction  in  the  hands  of 
many  inexperienced  workers,  was  difficult  to  operate  in  some  locali- 
ties. Thus  it  was  found  impossible  to  obtain  continuous  records  in 
the  dry  climate  of  the  Wasatch  Mountains  of  the  Manti  National 
Forest  in  central  Utah,  principally  on  account  of  the  connections 
and  joints  of  the  equipment,  all  of  which  occurred  outside  of  the 
water  reservoir.  Hail  storms  and  objects  carried  by  strong  wind 
were  often  so  severe  as  to  disjoint  or  break  the  more  delicate  equip- 
ment. In  connection  with  this  instrument,  it  is  understood  that  the 
automatic  mercury  valves  which  operate  to  prevent  the  water  ab- 
sorbed by  the  porous  cup  in  times  of  rain  from  entering  the  reservoir 
are  externally  situated.  This  makes  it  essential  to  have  all  valves 
very  tightly  connected  in  order  to  prevent  leakage.  Much  to  the 
satisfaction  of  those  who  have  used  this  instrument,  Shive  (165) 
has  described  one  so  modified  as  to  be  self-contained,  and  at  the 
same  time  to  reduce  to  the  minimum  the  liability  of  breakage  and 
the  difficulty  of  adjustment.  This  was  accomplished  by  eliminating 
jointed  mercury  valves  and  decreasing  the  leakability  and  breakage 
to  a  nominal  degree.     The  arrangement  of  the  different  parts  of  the 


158         BULLETIN  1059,  U.   S.  DEPARTMENT  OE  AGRICULTURE. 


instrument  is  shown  in  the  following  detailed  diagram  (fig.  4).  The 
description  of  the  self-contained  instrument,  which  is  particularly 
adapted  to  general  field  work,  is  quoted  from  that  prepared  by 
Strive. 

From  the  reservoir  (F)  two  glass  tubes  (A  and  B)  extend  upward  through  a  par- 
affined cork  stopper,  and  then  through  a  two-perforated  rubber  stopper  into  the  porous 
cup,  one  passing  to  the  tip  of  the  cup,  the  other  just  to  the  upper  surface  of  the  rubber 
stopper.  These  tubes  are  of  small  bore,  about  0.8  millimeter  inside  diameter.  Each 
is  bent  into  a  U  and  continued  upward  as  A7  and  B7.     One  (A7)  extends  through  the 

paraffined  cork  (B7)  extends  upward  6  centimeters  to  8  centi- 
meters and  is  stopper  and  ends  about  5  centimeters  above  it. 
The  other  again  bent  near  the  bottom  of  the  reservoir.  The  tube 
B  is  expanded  into  a  small  bulb  C  at  its  lower  extremity,  and  the 
tube  A7  is  expanded  into  a  similar  bulb  1  centimeter  to  2  centi- 
meters from  its  lower  end.  The  tube  E  is  about  1.2  centimeters 
in  diameter,  and  forms  a  shallow,  inverted  funnel  with  the  lower 
surface  of  the  paraffined  cork  stopper.  This  serves  to  conduct 
air  bubbles,  which  may  catch  on  the  undersurface  of  the  stopper 
in  filling  the  reservoir,  to  the  exterior.  The  tube  extends  5  cen- 
timeters above  the  cork  stopper  and  is  graduated  to  tenths  of 
cubic  [centimeters.  The  zero  point  on  the  tube  serves  as  a  zero 
point  in  filling  the  reservoir. 

To  install  the  instrument  the  paraffined  cork  stopper,  into 
which  the  tubes  A  A7,  B,  and  E  have  been  properly  lifted,  is 
tightly  pressed  into  the  mouth  of  the  reservoir  I".  A  sufficient 
amount  of  clean  mercury  is  allowed  to  fall  from  a  pipette  into  the 
openings  in  the  upper  end  of  each  of  the  tubes  A'  and  B  to 
form  a  column  5  centimeters  to  6  centimeters  high  in  tube  A', 
and  slightly  more  than  this  in  tube  B.  After  the  porous  cup  has 
been  placed  in  position  and  the  reservoir  filled  with  distilled 
water,  a  rubber  tube  is  attached  to  the  free  end  of  the  filling 
tube  A7,  and  gentle  suction  is  applied.  Water  rises  from  the 
reservoir  into  the  tube  B7,  at  the  same  time  that  the  mercury 
in  this  tube  is  drawn  into  the  bulb  C,  where  water  passes  freely 
and  rises  in  the  tube  B,  filling  the  porous  cup.  When  the  cup 
is  filled,  water  passes  into  the  tube  A  A',  the  mercury  in  this 
tube  having  been  drawn  into  the  bulb  L),  where  the  water  is 
allowed  to  pass  freely  and  escape  into  the  rubber  tube,  which  is 
then  removed.  The  mercury  in  the  bulbs  C  and  D  drops  back 
into  the  tubes  below.  To  prevent  water  loss  from  the  reservoir 
by  evaporation  through  the  tube  E  and  to  prevent  the  entrance 
of  water  through  this  tube  from  without  in  times  of  rain,  a  vial 
i  plac<  ver  the  end  of  this  tube.  A  suitable  vial  is  also  placed  over  the  end  of  the 
tube  A7  to  exclude  dirt.     The  instrument  is  now  ready  for  operation. 

To  replace  the  cup  with  a  new  one,  it  is  only  necessary  to  remove  the  old  cup  from 
its  support  and  to  place  the  new  cup  into  position,  after  which  suction  is  applied 
to  the  tube  A7,  as  in  installing. 

As  water  evaporates  from  the  surface  of  the  cup,  the  mercury  rises  in  tube  A  and 
falls  in  tube  A7,  coming  to  rest  with  the  mercury  level  in  A,  slightly  higher  than  in 


B 


1 


B' 


Fig.  4.— Shivc's  non- 
absorbent  porous 
cup  atmometer. 


RESEARCH  METHODS  IN  STUDY  OF  FOREST  ENVIRONMENT.      159 

A7,  the  difference  in  the  height  of  the  two  columns  depending  upon  the  height  of 
the  cup  above  the  water  level  in  the  reservoir.  The  mercury  in  B  W  is  drawn  into 
the  bulb  0,  where  water  rising  from  the  reservoir  is  freely  allowed  to  pass,  supplying 
in  the  usual  way  the  water  lost  from  the  surface  of  the  cup.  The  mercury  columns 
in  the  tube  A  A/  and  B  B/  remain  in  equilibrium  in  the  position  indicated  in  the 
figure,  so  long  as  the  water  loss  from  the  surface  of  the  cup  by  evaporation  equals  or 
exceeds  the  absorption  from  without  by  any  part  of  its  surface.  In  times  of  rain, 
when  the  water  loss  from  the  surface  of  the  cup  by  evaporation  is  less  than  the  absorp- 
tion from  without,  the  automatic  mercury  valves  become  reversed.  The  mercury 
column  falls  in  A  and  rises  in  A'  at  the  same  time  that  the  mercury  in  bulb  C  drops 
into  the  tube  below  and  rises  in  the  tube  B/  (the  height  to  which  the  mercury  rises 
in  this  tube  depending  upon  the  height  of  the  cup  above  the  water  level  in  the  reser- 
voir), thus  effectually  preventing  water  from  entering  the  reservoir  from  this  direction. 
The  readings  obtained  give  the  actual  evaporation  minus  the  error  introduced  by 
the  volume  change  required  for  the  operation  of  the  mercury  valves,  the  value  of  the 
error  thus  introduced  depending  upon  the  number  of  complete  reversals  of  the  valves. 

Standardization . 

The  Livingston  porous-cup  atmometers,  whether  to  be  used  with 
or  without  the  Shive  nonabsorbing  apparatus,  should  be  obtained  in 
the  standardized  form,  which  insures  comparability  of  the  results 
obtained  with  different  instruments  and  by  different  investigators. 
This  standardization  of  the  instruments  is  one  of  the  strongest 
features,  making  it  possible,  for  the  first  time,  for  different  investi- 
gators to  speak  of  evaporation  in  common  terms.  Since  the  stand- 
ardizing can  not  be  readily  accomplished  in  the  field,  it  is  sufficient 
to  state  that  it  is  very  carefully  done  at  the  Johns  Hopkins 
laboratory,  where  each  new  instrument  is  compared  with  one  or 
more  tried  instruments.  Assuming  the  rate  of  evaporation  for  a  cer- 
tain time  to  be  100  in  a  standard  instrument,  the  coefficient  of  an  in- 
strument which  in  the  same  period  evaporated  150  units,  would  be 
0.67.  In  short,  the  coefficient  given,  for  any  instrument  which  has 
been  " standardized,"  is  the  amount  by  which  all  records  of  that 
instrument  should  be  multiplied,  to  reduce  them  to  a  standard  basis. 

Computation  of  field  results. 

In  computing  the  field  results  to  a  standard  basis,  all  cups  used  for 
a  few  days  or  weeks  are  restandardized.  Provided  the  new  evapo- 
rating coefficient  is  found  to  differ  from  the  original,  the  probable 
average  coefficient  during  regular  cycles  or  periods  is  calculated. 
It  has  been  shown  experimentally  that  the  change  in  the  evaporation 
coefficient  takes  place  gradually  and  uniformly  during  the  period  of 
operation  (159).  Thus,  assume  that  a  given  atmometer  cup  having 
an  initial  coefficient  of  0.62  was  exposed  in  a  habitat  for  a  period  of 
four  weeks  during  which  it  was  read  three  times;  namely,  at  the 


160  BULLKT1N    L059,    I.    S.    DEPARTMENT    OF    AGRICULTURE. 

end  of  the  first,  the  second,  and  the  fourth  week.  Upon  standardi- 
zation niter  tlie  Last  reading  (that  is,  after  the  end  of  the  fourth 
week',  the  cup,  suppose,  had  a  coefficient  of  0.70  in  place  of  the 
original  coefficient  of  0.62.  A  variation  of  0.08  is  therefore  to  be 
distributed  uniformly  over  the  four-week  period.  At  the  end  of  the 
first  week  the  assumed  coefficient  is  0.62;  at  the  end  of  the  second 
week  it  is  0.60:  and  at  the  end  of  the  fourth  week,  0.70.  According 
to  the  above  assumption,  the  average  coefficient  for  the  first  week 
would  be  0.63;  for  the  second  week,  0.65,  etc.  Assuming,  then, 
that  the  reading  at  the  end  of  the  first  period — that  is.  at  the  end  of 
a  week— was  500  cubic  centimeters,  by  reducing  to  standard  units 

,      .         ,.       500X63     Q1_ 
we  have  the  standard  reading  -  -^00"    =315  c.  c. 

/-.'  posure. 

This  instrument  will  operate  over  a  long  period,  the  time  de- 
pending upon  the  size  of  the  reservoir  and  the  exposure  to  the  evap- 
orating power  of  the  air.  For  this  reason  the  instrument  may 
be  exposed  upon  remote  habitats  where  weekly,  bimonthly,  or  even 
monthly  readings  may  be  made.  Records  of  this  kind,  of  course, 
are  less  conclusive  than  those  made  at  more  frequenl  period-,  but 
nevertheless  have  a  high  value  for  comparative  consideration. 
Readings  taken  only  bimonthly  or  so  are  accurate  because  of  the  per- 
manent, rain-correcting  adjustment  of  the  instrument,  a  point  which 
has  already  been  brought  out  in  detail. 

Since  the  instrument  is  compact  and  light  in  weight,  it  is  possible 
to  make  the  exposure  practically  wherever  desired.  If,  for  example. 
the  investigator  desires  to  determine  the  evaporation  rate  near  the 
surface  of  the  ground  amidst  low-growing,  herbaceous  vegetation  or 
arborescent  seedlings,  the  reservoir  is  sunk  in  the  soil  so  that  the 
evaporating  surface  of  the  atmometer  is  exposed  at  the  height  de- 
sired. In  cases  where  the  evaporation  rate  is  desired  at  various 
heights,  the  instruments  may  be  placed  at  definite  vertical  elevations 
on  light  supports  running  out  horizontally  from  a  common  vertical 
beam. 

The  principal  disadvantages  mentioned  with  respect  t<>  the  Piche 
evaporimeter  have  been  eliminated  in  the  construction  of  the  porous 
cup  with  its  nonabsorbing  attachments.  It  is  of  simple  construction, 
not  extremely  expensive,  occupies  a  small  space,  and  may  be  equipped 
with  a  reservoir  which  will  keep  it  in  operation  for  an  indefinite 
period.  Wind  does  not  disturb  it.  and  small  animals  and  insects 
have  no  effect  upon  its  operation.  The  disadvantages  are  that  the 
bottle,  connecting  tubes,  and  especially  the  delicate  nonabsorbing 
apparatus  are  rather  easily  broken  by  inexperienced  hand-,  or  by 
roving  animals  such  as  domestic  stock  and  deer.     A  -till  more  serious 


RESEARCH  METHODS  IN  STUDY  OF  FOREST  ENVIRONMENT.      161 

difficulty  is  the  sensitiveness  of  the  instrument  to  frost;  the  slightest 
freezing  of  the  water  in  the  connecting  tubes  drives  the  mercury  out 
of  the  valves  and  generally  bursts  the  tubes,  which  are  the  most 
expensive  part  of  the  instrument.  Even  when  the  air  temperature 
is  not  below  32°  F.,  but  the  wet-bulb  temperature  is  below  freezing, 
ice  accumulates  on  the  cup  in  large  quantities.  In  short,  the  instru- 
ment is  worthless  when  the  temperatures  approach  freezing. 

Other  disadvantages  are  the  necessity  for  frequent  restandard- 
izing,  and  the  occasional  failure  of  the  cup  to  draw  moisture  from 
the  reservoir  as  fast  as  evaporated.  This  permits  the  entry  of  air 
and  reduces  the  evaporating  surface,  entirely  vitiating  the  results 
until  the  cup  is  refilled. 

Forest  Service  evaporimeter. 

To  fill  the  need,  in  forest  studies,  for  a  substantial,  essentially  in- 
destructible evaporimeter,  operating  as  well  in  winter  as  in  summer 
(since  it  can  not  be  conceded  that  biological  activity  ceases  with  the 
first  occurrence  of  frost),  Bates  (151)  has  designed  a  metallic,  " inner 
cell'1  evaporimeter.  In  addition  to  these  desired  practical  qualities, 
it  was  conceived  that  an  evaporimeter  might  more  closely  resemble 
the  leaf,  and  thereby  might  show  a  closer  correlation  with  plant 
transpiration,  by  having  the  evaporating  body  somewhat  protected 
from  air  currents.  Evaporation  takes  place  in  the  leaf,  not  largely 
on  its  surface;  and,  while  transpiration  is  accelerated  by  wind  move- 
ment, the  latter  can  not  have  the  direct  action  upon  the  water  in  the 
leaf  and  its  formation  into  vapor  that  it  has  upon  a  fully  exposed 
surface. 

The  correctness  of  this  principle  has  been  demonstrated  by  com- 
parative tests  of  evaporation  and  transpiration  under  a  variety  of 
conditions.     The  evaporimeter  may  be  briefly  described  as  follows: 

The  tank  or  reservoir  has  a  capacity  of  about  450  cubic  centimeters, 
sufficient  for  a  week's  operation  under  extreme  conditions.  It  is 
seamless  and  is  not  ordinarily  injured  by  freezing.  It  is  protected 
by  an  outer  shell  of  polished  metal,  which  insulates  it  both  against 
temperature  changes  and  against  direct  radiation. 

Out  of  the  tank  rises  a  stem  a  few  inches  long  and  one-half  inch 
in  diameter,  carrying  the  feed-wick,  which  is  a  piece  of  linen  rolled 
into  cylindrical  form  with  the  threads  " drawn'1'  at  one  end.  At  the 
top  of  the  stem  this  wick  is  flattened  out  to  make  a  contact  with  the 
evaporating  wick.  The  evaporating  wick  is  a  flat,  circular  piece  of 
linen  having  an  area  of  100  square  centimeters.  It  rests  upon  a 
perforated  metal  disk  of  the  same  size,  the  perforations  aggregating 
an  area  of  5  square  centimeters,  and  designed  to  simulate  the  stomata 
10163— 22— Bull.  1059 11 


162         BULLETIN   1059,   U.   S.   DEPARTMENT   OF   AGRICULTURE. 

of  the  under  side  of  a  leaf.  All  vapor  formed  must  escape  through 
these  perforations.  The  disk  is  firmly  attached  to,  and  flush  with, 
the  upper  end  of  tin1  stem. 

(her  the  wick  is  a  cover  only  slightly  larger  than  the  disk,  whose 
flanged  edge  extends  down  over  the  edge  of  the  disk.  This  is 
held  down  by  two  screws,  which  engage  the  flanged  edge.  The 
cover  is  flat,  but  seamless,  and  completely  excludes  rain  or  snow.  It- 
surface  is  finished  with  nickel  and  an  instrument  having  this  polished 
surface  absorbs  practically  no  radiant  energy  and  is  called  a  "  shade' 
instrument.  To  obtain  the  effects  of  radiation,  an  instrument  whose 
cover  has  been  coated  dead-black  is  used.  The  "shade"  instrument 
is  dow  entirely  abandoned,  since  it  has  been  seen  that  the  difference 
between  the  two  is  not  a  measure  of  sunlight  intensity,  but  a  measure 
of  the  additional  effect  of  sunlight  in  producing  evaporation.  This 
effect  can  not  under  an}'  circumstances  be  ignored  in  ecological 
studies. 

The  tests  which  have  been  made  show  that  the  looses  from  the 
blackened  instrument  of  this  type  follow  more  closely  those  from 
potted  trees,  under  a  great  variety  of  atmospheric  and  solar  condi- 
tions, than  do  the  losses  from  any  other  type  of  instrument  at 
present  available.  The  instrument  has  also  shown  itself  remarkably 
free  from  annoying  characters,  and  responsive  t<>  all  degrees  of 
evaporation  stress.  It  may  be  considered,  however,  something  of  a 
disadvantage  that  the  amount  evaporated  i-  relatively  small.  The 
losses  for  short  periods  may,  therefore,  only  he  determined  by  very 
precise  weighing. 

OBSERVATIONS. 

With  the  Forest  Service  evaporimeter  the  daily  observations  con- 
sist in  weighing  the  instrument,  usually  on  an  inexpensive  balance 
such  as  the  Harvard  trip  scale.  Refilling  is  undertaken  as  -ft  en  as 
necessary  to  maintain  between  100  and  200  cubic  centimeters  of  water 
in  the  tank.  The  "closing"  weight  and  "refilled"  weight  are  en- 
tered on  the  field  observation  form,  together  with  the  number  of 
the  instrument.  Computations  of  losses  are  usually  made  when 
entering  the  data  on  Form  8,  and  the  correction  in  accordance  with 
the  calibration  of  the  instrument  is  usually  applied  only  to  the  total 
losses  for  10-day  periods. 

With  porous  cups  the  daily  or  periodic  observations  will  usually 
ist  m  an  entry  of  the  amount  of  water  required  to  fill  the  reser- 
voir to  datum  level.  A  graduate  is  taken  into  tin-  field  to  measure 
this  amount. 

In  the  case  of  the  Piche  evaporimeter,  losses  are  calculated  from 
the  readings  of  the  graduated  reservoir.  Note  should  always  be 
made  of  observed  overflowing  or  drying-up  of  the  filter  paper  or 


RESEARCH  METHODS  IX  STUDY  OF  FOREST  ENVIRONMENT.      163 


evaporating  surface,  and  the  probable  correction  due  to  these  fail- 
ures of  the  instrument  should  in  all  cases  be  estimated  and  entered 
in  the  " Corrections "  column  of  the  field  form  "Daily  Observations." 
Interpolations  of  this  character  should  be  indicated  by  asterisks  on 
the  "Humidity,  Wind,  and  Evaporation"  form. 


TABULATION". 


The  tabulation  of  evaporation  data  on  the  "Humidity,  Wind,  and 
Evaporation'  form  consists  in  setting  down  all  of  the  recorded 
weights  of  the  evaporimeters,  or  graduate  readings,  and  the  losses 
computed  therefrom.  In  the  record  for  porous  cup  atmometers, 
there  will  usually  be  but  one  entry  for  each  observation,  though 
these  instruments  may  also  be  handled  on  a  weight  basis  more  satis- 
factorily than  on  a  volume  basis.  The  corrections  for  calibration 
are  made  usually  on  the  sums  for  10-day  periods. 

Form  Z 

Daily  observation  at 


Date  of  station  No     . 

Time.                                     

Datum: 

Maximum  temperature  air 

Minimum  temperature,  air. 

(  urrent  temperature,  air              

Current  thermograph  pen.  .  . . 1 

Psvehrometer: 

Dry  bulb 

We1  bulb  

Anemometer  dial 

Soil  temperature: 
Surface 

1  foot .  .  . 



2  or  4  feet 

(  urr.  thermograph 

Evaporimeter — Number: 
Closing  reading  . 



Refilled  to 

Amount  since  last  ob. . .             

E  vaporimeter — X  umber: 
("losing  reading 

Refilled  to . 

Amount  since  last  ob 



Current  precipitation: 
Water,  inches. 

Snow,  inehes 

i 

Snow  on  the  ground,  inches              

Frost  (indicate  by  X  or  XX           .    

Barometer,  inches 

Phenology : 
Tree  1 

Tree  2 

■ 

Tree  3 

■ 

Date  or  month 


164  BULLETIN    1050,   U.   S.   DEPARTMENT   OF   AGRICULTURE. 

The  annual  summary  of  evaporation  should  be  made  on  the 
"Summary"  form  and  should  consider  the  total  evaporation  by 
decades,  months,  the  year,  and  the  growing  season.  It  may  also  be 
desirable  to  record  the  maximum  rate  noted  during  each  month  or 
decade. 

DIRECT  TRANSPIRATION"  METHODS. 

In  evaporation  studies  connected  with  plant  life  the  chief  purpose 

bo  obtain  in  as  simple  a  term  as  possible  a  measure  of  the  habitat 

conditions,  as  they  may  affect  transpiration.     The  response  of  the 

plant  to  these  conditions  will  be  governed  by  physical  facts  that  can 

he  fairly  well  comprehended,  and  by  biological  conditions  which  are 

-.till  more  or  less  obscure.     While,  then,  transpiration  studies  may 

be  made  in  lieu  of  evaporation  studies,  it  will  be  far  more  profitable 

to  consider  the  one  as  supplementing  the  other,  giving  an   insight 

into  plant  functioning  which  can  only  he  obtained  a-  observations  are 

reduced  to  terms  of  well-known  physical  laws.     One  of  the  means 

of  determining  the  intensity  of  transpiration  i-  by  the  aid  of  coball 

chloride  paper. 

( 'obalt-chloridi  method. 

Although  the  actual  amount  of  water  transpired  by  a  plant  can 
not  be  ascertained  bv  means  of  the  standardized  cobalt*chloride 
paper,  this  method  is  particularly  adapted  to  field  use  where  merely 
relative  rates  of  transpiration  are  desired,  and  for  showing  varia- 
tions in  rate  with  changes  in  environmental  condition-.  It  is  hardly 
necessary  to  point  out  that  it  would  be  almost  impossible  to  apply 
to  needle  leaves.  The  method  depends  upon  the  fact  that  paper  im- 
pregnated with  a  weak  solution  of  cobalt  chloride  i-  blue  when  dry. 
but  when  exposed  to  moisture  gradually  turns  pink.  Specially  pre- 
pared ''tripartite  cobalt-chloride  paper  slips'  may  be  purchased. 
These  -lips  are  made  up  of  three  small  st rip-  of  paper  a  deep  blue 
standard,  a  light  blue  standard,  and  between  them  a  -trip  of  cobalt 
chloride  paper.  In  practice  the  strip  is  heated  over  a  small  flame 
or  heated  surface  until  the  cobalt  chloride  strip  is  of  a  more  intense 
blue  than  either  standard.  It  is  then  applied  to  the  surface  of  the 
leaf  under  consideration  and  held  by  small  glass  clip-.  When  the 
cobalt  chloride  strip  fades  so  that  it  matches  the  bluest  standard,  the 
time  is  noted.  When  it  fades  further  and  matches  the  light  bine 
standard,  the  time  is  again  noted  and  the  elapsed  time  i<  recorded. 
This  process  should  be  repeated  several  time-  to  assure  a  good 
average.  A  thermometer  should  also  be  hung  amidst  the  foliage  to 
give  the  temperature  of  the  leaves. 

The  time  taken  for  the  cobalt-chloride  paper  to  make  the  change  m 
color  between  the  two  standards  is  the  measure  of  the  rate  of  Iran- 


RESEARCH   METHODS  IX  STUDY  OF  FOREST  ENVIRONMENT.      165 

piration  of  the  leaf.  In  order  to  refer  this  to  a  definite  standard,  it  is 
compared  with  the  time  required  by  the  strip  to  make  the  same  change 
over  a  free-water  surface  blanketed  by  one  millimeter  of  air  at  a  tem- 
perature the  same  as  the  leaf.  The  reaction  time  of  each  slip  must 
therefore  be  ascertained  under  standard  conditions.  The  apparatus 
for  making  these  standardizations  is  described  in  detail  bv  Livings- 
ton  and  Shreve  (161). 

It  would  obviously  be  a  great  labor  to  determine  the  reaction  time 
of  each  slip  for  every  temperature  likely  to  be  used;  but  this  is  un- 
necessary, because  if  the  time  is  determined  for  one  temperature  it 
can  be  easily  derived  for  any  other,  since  it  is  inversely  proportional 
to  the  pressures  of  saturated  aqueous  vapor  at  the  different  tem- 
peratures. 

The  chief  objection  to  the  method  is  the  large  personal  equation  in- 
volved. The  fading  of  tin4  slip  is  very  slow  indeed,  and  no  perceptible 
change  may  occur  for  many  seconds.  Practically  the  system  seems  to 
work  out  well  and  yields  much  more  consistent  results  than  potom- 
eters.  It  is  easily  used  in  the  field  and  in  all  kinds  of  sites,  and 
appears  to  be  a  valuable  addition  to  field  research  methods. 

Method  *  i'  •  tcised  twigs. 

In  measuring  transpiration  by  means  of  cut  twigs  placed  in  potom- 
eters.  the  actual  amount  of  water  given  off  per  unit  of  leaf  surface 
can  he  determined. 

The  apparatus  used  consists  of  a  flask  closed  with  a  two-hole  rub- 
ber stopper,  in  one  hole  o[  which  is  inserted  a  glass  tube  bent  at  right 
angles;  in  the  other  the  twig  is  sealed.  The  horizontal  section  of  the 
glass  tube  is  graduated  so  that  it  may  be  used  to  measure  the  water 
removed  by  the  twig,  and  is  drawn  to  a  fine  point  at  the  end  to  mini- 
mize evaporation  from  the  end  of  the  tube.  The  twigs  must  be  cut 
off  under  water  and  the  ends  must  be  kept  continuously  wet,  or  large 
variations  in  transpiration  will  occur.  The  sealing  into  the  potom- 
eters  must  be  very  carefully  done  also,  as  the  least  leak  will  totally 
vitiate  results.  This  method  is  particularly  useful  when  it  is  de- 
sirable to  determine  transpiration  of  several  species  or  in  several  sites 
simultaneouslv.  The  results  tend  to  be  erratic,  however,  and  ac- 
cordingly  the  determinations  should  be  made  in  duplicate,  at  least  to 
avoid  gross  errors.  The  same  twigs  can  only  be  used  for  a  short  time, 
after  which  fresh  ones  must  be  obtained. 

To  determine  the  leaf  area,  as  must  be  done  to  reduce  the  transpi- 
ration to  amount  per  unit  surface,  the  simplest  method  is  to  cut  a 
number  of  pieces  of  known  area  with  a  cork  cutter,  or  preferably  a 
centimeter  punch,  weigh  them,  and  then  weigh  the  residue  of  the 
leaves  and  figure  their  area  proportionately. 


166  BULLETIN    1059,    U.    S.    DEPARTMENT    OF    AGRICULTURE. 

Method  of  potted  plants. 

The  mot  hod  of  determining  the  transpiration  rate,  either  in  the 
laboratory  or  under  a  variety  of  field  conditions,  by  means  of  potted 
plants,  i-  theoretically  the  most  reliable,  since,  if  the  potted  plants 
are   kept    in    ;i    healthy  condition   and   the   moisture   supply   in    the 

I  i-  properly  regulated,  a  very  near  approach  to  natural  growth 
conditions  may  be  obtained.  Unquestionably,  the  rate  of  transpira- 
tion is  limited  by  the  moisture  supply.  Where  tests  of  this  char- 
acter arc  made  in  the  field  the  moisture  content  of  the  pots  may 
at  all  times  he  similar  to  that  of  the  native  soil  at  the  point  studied, 
allowance  being  made,  of  course,  for  differences  in  physical  prop- 
erties of  the  two  soils,  if  any  exist.  A  much  simpler  method  is  to 
maintain  the  pots  constantly  at  a  standard  moisture  content  known 
to  he  near  the  optimum.  Thorn  and  Holtz  (168)  and  Kiosselbach 
L56)  have  shown  that  the  maximum  transpiration  occurs  when  the 
<n\\  is  about  half  saturated.  This  may  be  the  result  of  aeration,  or 
it  may  be  that  "about  half  saturation"  corresponds  to  the  " critical 
moisture  content"  of  Cameron  and  Gallagher  (117'.  in  which  <•;: 
it  is  more  likely  to  be  a  question  of  osmosis. 

A  method  of  treating  plants  that  seems  especially  adapted  t<» 
good-sized  trees  is  that  of  Briggs  and  Shantz  (153,  154),  having 
been  employed  to  determine  the  amount  of  water  used  by  various 
agricultural  crop  plants  under  semiarid  conditions  at  Akron.  Colo. 
These  tests  were  made  on  a  large  scale,  with  all  arrangements  de- 
signed for  outdoor  exposure.  The  pots  were  galvanized  ash  cans, 
through  the  covers  of  which  the  stems  of  the  plant-  were  made  t«» 
extend  after  growth  had  become  well  established.  The  weighing 
of  such  pots  was  laborious,  of  course,  and  required  the  use  of  a 
traveling  crane  arrangement,  by  means  of  which  the  can-  were 
lifted  to  the  scales  for  each  weighing.  A  very  similar  arrangement 
would  appear  practicable  for  determining  transpiration  from  tree 
specimens,  until  they  had   attained   the  height    of   at    least     1    or  5 

feet. 

For  the  convenient  handling  of  forest  tree  seedlings  up  to  •">  or 
•'»  years  of  age,  a  galvanized  iron  can.  4  inches  in  diameter  and  10 
inches  deep,  has  been  used  by  Bates  (105).  This  can  is  soldered 
and  there  are  no  perforations  in  the  bottom   through  which  water 

l  escape.  Before  the  seedlings  are  potted,  a  2-inch  clay  flower- 
pot is  inverted  in  the  bottom  of  each  can.  Through  the  whole  in  the 
base  of  the  flowerpot  a  glass  tube  of  small  bore,  bent  with  two 
right  angles  so  that  the  main  part  of  the  tube  rests  against  the 
wall  of  the  galvanized  can,  is  inserted.  It  extends  slightlv  above 
the  wall  of  the  can.  This  glass  tube  serves  to  \\hu\  water  into  the 
porous  pot,  whence  it  is  readily  diffused   through   the  soil,   and   at 


RESEARCH  METHODS  IN  STUDY  OF  FOREST  ENVIRONMENT.      167 

the  same  time  permits  the  entrance  of  air,  which  may  be  obtained 
by  the  roots  through  the  walls  of  the  pot,  insuring  their  mainte- 
nance in  a  healthy  condition. 

For  the  best  distribution  of  moisture  in  the  soil,  it  is  desirable 
that  a  second  tube,  or  a  tee  connection  on  the  longer  one,  should  be 
placed  so  as  to  extend  about  an  inch  below  the  soil  surface.  Since 
there  will  be  some  escape  of  vapor  from  each  of  these  tubes,  a  pot 
similarly  prepared,  but  containing  no  tree,  should  be  run  as  a 
measure  of  such  loss,  under  the  various  conditions  to  which  the 
trees  are  exposed. 

The  weight  of  the  can,  pot,  and  glass  tube  is  first  obtained.  The 
seedling  to  be  potted  is  then  weighed  with  the  minimum  of  exposure 
to  the  air.  The  seedling  is  then  placed  in  the  can,  which  is  filled 
with  moderately  dry  soil,  and  the  weight  of  the  whole  is  obtained 
immediately  after  potting,  following  which  water  may  be  given  to 
the  plant.  Having  now  the  weight  of  the  air-dry  soil  which  has 
been  placed  in  the  pot,  its  net  oven-dry  weight  may  readily  be  com- 
puted, after  drying  small  samples  of  the  same  soil;  and  from  this 
net  weight  may  be  calculated  the  amount  of  water  which  the  pot 
should  contain  at  all  times  to  maintain  a  moisture  equal  to  50  per 
cent,  let  us  say,  of  the  saturation  capacity  of  the  soil.  Once  the  soil 
is  well  settled  by  watering,  the  top  of  the  can  is  sealed  with  a  mix- 
ture of  paraffin  and  vaseline.  A  measured  amount  is  used,  so  that 
the  weight  of  this  substance  may  also  be  included  in  the  total  weight 
which  the  outfit  should  show  at  the  desired  moisture  condition. 

Knowing  the  weight  which  the  outfit  should  have  at  a  certain 
moisture  condition,  the  simplest  method  of  measuring  the  transpira- 
tion is  to  put  the  can  on  one  side  of  the  scales  and  the  desired  weights 
on  the  other  side,  and  to  inject  water  from  a  burette  until  a  balance 
is  obtained.  The  water  may  alternately  be  injected  through  the  long 
and  the  short  glass  tubes. 

The  transpiration  results  will  be  most  expressive  if  given  in  terms 
of  transpiration  per  unit  of  leaf  area;  but  since,  with  coniferous  seed- 
lings, the  determination  of  leaf  area  with  any  precision  is  next  to 
impossible,  the  plan  of  computing  the  loss  per  unit  of  weight  of  the 
plant  may  be  considered. 

Where  a  number  of  plants,  even  though  of  the  same  species  and 
grown  under  the  same  conditions,  are  to  be  placed  in  potometers  for 
transpiration  study  under  a  variety  of  field  conditions,  the  plants 
should  by  all  means  be  calibrated  under  the  same  conditions  before 
being  distributed,  since  extremely  great  variations  in  individuals 
seem  to  be  inherent. 


168  BULLETIN    L059,    U.    S.   DEPARTMENT   OF   AGRICULTURE. 

Instruments  (Approximate  Prices). 

Piche  evaporimeter,  modifications  used  by  Weather  Bureau:  graduated  to  0.2 
dc   centimeter,   capacity  about  40  cubic   centimeters.     Supplied   with 

ilate  9  centimeters  in  diameter,  and  1  dozen  paper  disks.  No.  345  6 |6.  50 


Paper  disks  for  evaporimeter,  per  dozen.  Xo.  345  c 


Li  >.  ;  i  porous-cup  atmometers : 

Natural  cups,  cylindrical  with  smoothly  ground  surface,  for  general  pur- 
poses  -v •)U 

i  loated  cups,  glazed  at  base,  with  permanent  numbering 60 

Spherical  cups,  standardized 1.  00 

Shave's  nonabsorbing  attachment 6.  00 

Foresl  Service  evaporimeters,  equipped  with  wicks,  blackened  and  calibrated.    1 1.  00 
Evaporating  pans,  10  inches  deep,  4  feet  in  diameter,  constructed  of  No.  22  gauge 
B.  W.  G.  galvanized  iron;  can  be  constructed  locally  or  secured  through  the  I'nited 
States  Weather  Bureau. 
Cobalt-chloride  paper  slips,  "tripartite." 
•  lips,  glass,  for  attaching  cobalt-chloride  paper  to  lea 

PHENOLOGY. 

The  prevailing  idea  of  ecology  as  a  science  through  which  the 
mysteries  of  plant  and  animal  life  may  be  solved  merely  by  measur- 
ing the  environment  in  more  or  less  exact  terms,  is  gradually  giving 
way  to  a  conception  of  ecology  as  a  phase  of  physiology.  In  the 
preceding  sections  it  has  been  attempted  to  bring  out  the  concept 
of  physiology,  at  least  in  its  broader  aspect,  as  a  basis  for  those 
observations  and  measurements  which  are  commonly  associated  wit  h 
ecological  studies.  Unless  the  nature  of  physiological  reaction-  in 
the  plant  is  understood  in  a  general  way,  the  stimuli  which  are  im- 
portant to  plant  life,  can  not  be  correctly  measured;  that  is,  the 
environment  can  not  be  measured  in  terms  which  are  expressive. 
On  the  other  hand,  each  such  effort,  by  reducing  the  stimulus  to 
physical  terms,  permits  a  little  better  understanding  of  the  activ- 
ities of  the  plant,  by  placing  them  more  nearly  on  a  physical  basis. 
Thus  physiology  and  ecology  must  advance  side  by  side,  or,  bo 
use  a  crude  simile,  like  the  tread  on  a  caterpillar  tractor,  in  which 
each  segment  is  in  turn  brought  forward  to  a  point  where  it  may 
perform  its  temporary  function. 

Now,  in  fact,  phenology  is  ecology  as  applied  to  function-  of 
plants  and  animals  which  are  more  or  less  regularly  periodic  or  sea- 
sonal in  character.  Unfortunately,  much  good  effort  has  been  wasted 
on  phenological  observations,  particularly  in  connection  with  trees- 
wasted,  because  no  correlation  was  attempted  between  the  phenomena 
of  growth  and  any  other  condition  except  time,  calendar  dal 
Again,  effort  has  been  wasted  because,  while  correlation  between 
growth  and  climatic  conditions  was  attempted,  the  observations  on 


RESEARCH  METHODS  IN  STUDY  OF  FOREST  ENVIRONMENT.      169 

growth  particularly  were  too  crude;  no  means  was  at  hand  of 
determining  closely  when  growth  began,  how  rapidly  it  proceeded, 
when  it  ceased.  In  consequence,  phenological  observations  have 
fallen  into  disrepute. 

The  object  of  these  conclusions  is  to  suggest  that,  after  all,  in  the 
study  of  ecology  there  is  nothing  more  important  than  the  behavior 
of  the  plant  itself,  its  reactions  at  various  times  and  seasons;  in  other 
words,  phenology  in  the  fullest  sense.  Otherwise,  ecological  studies 
may  as  well  be  left  to  climatologists  and  soil  physicists. 

How  then  may  observations  on  the  plant  be  made  worth  while? 
The  observable  external  phenomena  which  accompany  a  reaction  to 
certain  environmental  conditions  must  be  measured  more  precisely 
than  in  the  past  if  they  are  to  serve  any  useful  purpose.  There  is 
room  here  for  instrumental  development,  quite  as  much  as  in  the 
study  of  the  environment.  The  field  has  been  even  more  neglected. 
Again,  there  is  opportunity  for  studying  changes  in  the  plant  through 
internal  physical  and  chemical  conditions.  This  brings  this  study 
directly  into  the  field  of  physiology,  which  can  not  be  covered  further 
than  has  already  been  done.  Finally,  experimental  physiology, 
or  the  study  of  reactions  to  a  limited  change  in  environment,  most 
of  the  conditions  being  stable  and  under  control,  is  necessarily  a 
laboratory  study.  The  nature  of  the  studies  involved  has  been 
indicated  in  preceding  discussions,  especially  in  connection  with 
light  and  soils  studie-.  They  may  have  a  very  useful  result  in  show- 
ing how  better  to  study  conditions  in  the  field,  but  they  do  not  take 
the  place  of  field  observations.  The  reactions  produced  by  changing 
one  factor  while  other  conditions  are  more  or  less  perfectly  con- 
trolled, may  not  be  at  all  the  same  as  in  the  field  where  all  the  factors 
vary  synchronously.  After  all,  then,  the  problem  for  the  ecologists 
simmers  down  to  one  of  determining  plant  reaction  in  the  field  as 
closely  as  possible. 

In  the  past  the  plant  society  has  perhaps  been  used  too  much  as 
an  index  to  reactions;  that  is,  the  effect  of  environmental  conditions 
has  been  judged  almost  wholly  by  end  results,  in  which  competition 
plays  an  important  part.  A  plant  is  either  absent  from  a  given  site, 
occasional,  abundant,  moderately  successful,  or  vigorous  and  domi- 
nant. From  this  is  judged  the  extent  to  which  the  species  is  favored 
or  inhibited  by  the  environmental  conditions  that  have  been  meas- 
ured. This  method  is  altogether  too  gross  and  undoubtedly  has 
led  to  a  great  many  erroneous  conclusions.  A  great  deal  more  is  to 
be  learned  as  to  the  requirements  of  different  species  by  closer 
observation  of  individuals. 


170         BULLETIN    1059,   U.   S.  DEPARTMENT   OF  AGRICULTURE. 

EXTERNAL   FIELD    OBSERVATIONS. 

At  each  station  where  the  environment  is  being  studied,  one  or 
more  individual  trees  should  be  permanently  marked  and  numbered. 
and  their  condition  should  be  a  matter  of  daily  observation.  The 
number  of  trees  chosen  will  depend  upon  the  composition  of  the 

and.  Where  even-aged  stands  of  one  species  are  studied,  probably 
two  representative  trees  of  the  dominant  and  codominant  classes 
will  be  sufficient.  In  all-aged  stands,  at  least  three  trees  of  various 
above  reproduction  should  be  included  and  two  sets  of  such 
trees  if  the  stand  contains  more  than  one  prominent  species,  or  if  the 
site  is  being  studied  from  the  standpoint  of  more  than  one  species. 
In  studies  of  reproduction  conditions,  the  smallest  seedlings  available 
should  be  under  observation. 

In  these  external  observations,  Forest  Service  Form  416  may  be 
used  as  a  guide,  since  it  covers  comprehensively  the  ordinary  phe- 
nologies! observations.  A  reduced  copy  of  this  form  may  well  be 
kept  in  the  field  notebook,  and  the  field  observations  may  be  given 
in  the  form  of  numbers  corresponding  to  the  captions  of  the  "Phe- 
nological  Observations'7  form.  For  example.  ul'  would  be  used  to 
indicate  that  buds  were  beginning  to  swell. 

As  has  been  said,  ocular  observations  on  these  common  phenomena 
of  growth,  especially  at  the  actual  beginning  of  tree  growth  in  the 
spring  and  its  termination  in  the  fall,  are  too  crude.  Some  method 
of  measuring  and  recording  the  actual  growth  from  day  to  day  is 
required.  A  number  of  auxometers  might  be  mentioned,  but  a 
meter  really  adapted  to  plants  in  the  field,  and  especially  the  exposed 
-waving  tops  of  trees,  has  not  been  produced,  so  far  as  the  writers 
know.     There  is  an  unlimited  field  here  for  invention. 

Form   H6. 

[U.  S.  Department  of  Agriculture,  Foresl  Service.] 

PHENOLOGICAL  OBSERVATIONS, 

Species 

Period  covered  by  observations 

Name  of  observer 

Residence 

<State-)  (County.)  "Jtowd 

General  character  of  country.- Mountains;  foothills;  plains;  river  valley;  seacoast 

■  it  nation  of  trees -Level;  slope  (north,  east,  west,  south  i;  hilltop;  riv<  r  bottom;  soil  sandy 
clayey,  heavy,  light,  deep,  shallow,  moist,  dry);  forest;  open  ground; 
(Please  check  the  words  which  apply  to  your  particular  locality  and  to  1  he  I  n  i 

Approximate  elevation  above  sea  level 

Location  of  nearest  Weather  Bureau  station. .... 

State  if  seasoD  was  wet  or  dry,  early  or  late,  etc 


RESEARCH    METHODS  IN  STUDY  OF  FOREST  ENVIRONMENT.       171 

Date.  Date. 

1 .  Swt  lling  of  buds 9.  End  of  leaf  falling 

2.  Bursting  of  buds 10.  Beginning  of  seed  ripening 

3.  Begin  n  ing  of  leafing  out 11 .  General  seed  ripening 

4.  General  leafing  out 12.  Beginning  of  seed  falling 

5.  Begin  n  ing  of  blossom  ing 13.  End  of  seed  falling 

6.  General  blossom  ing 14.  Quantity  of  seed 

7.  Chang,  in  color  of  foliage 15.  Quality  of  seed 

8.  Beginning  of  leaf  falling 

General  remarks 

Instructions  on  hack  of  this  form  should  be  followed  strictly. 

Under  the  name  " dendograph '  McDougal 22  has  designed  a  new 
instrument  for  measuring  and  recording  the  diameter  growth  of  tree 
stems.  This  instrument  is  bring  thoroughly  tried  out.  It  will 
probably  be  a  very  valuable  adjunct.  It  is  not  simple  in  construction 
or  operation,  however,  and  will  always  be  too  expensive  to  be  exten- 
sively used.  It  would  seem  that  a  beginning  must  be  made  in  a  more 
simple  way,  perhaps  through  circumferential  measurements,  even 
though  a  number  of  complicating  factors  must  be  taken  into  account, 
such  as  the  expansion  of  the  tree  and  of  the  tape  with  increased  tem- 
peratures. 

INTERNAL  OR  PHYSIOLOGICAL  OBSERVATIONS. 

Although  the  forester  is  prone  to  think  of  growth  as  the  major 
reaction  of  interest,  it  is  entirely  possible  that  there  may  be  positive 
gain  in  studying  the  more  fundamental  reactions  which  lead  up  to 
growth.  For  example,  in  the  conifers,  the  outward  evidences  of 
growth  may  disappear  by  midsummer,  so  far  as  height  accretion  is 
concerned:  yet  diameter  growth  continues  longer,  and  this  period 
of  relative  inactivity  is  of  great  importance  in  accumulating  a  reserve 
for  the  effort  of  the  following  season.  Is  it  not  logical,  therefore, 
that  the  growing  season  for  trees  should  be  considered  to  be  the 
entire  period  in  which  materials  for  growth  are  being  produced  ? 

Again,  while  growth  is  a  large  factor  when  competition  begins, 
the  critical  conditions  which  have  the  greatest  bearing  on  the  success 
of  the  individual  and  species,  and  thereb}^  affect  most  acutely  the 
character  of  the  plant  formation,  may,  in  the  case  of  all  perennial 
plants,  be  encountered  not  in  the  growing  season  but  in  the  dead  of 
winter.  Through  neglect  of  this  period  erroneous  conclusions  may 
again  be  reached  as  to  the  importance  of  various  conditions  in 
building  up  the  plant  formation. 

It  should  therefore  be  most  desirable  to  be  able  to  determine  the 
physiological  conditions  of  the  plant  frequently  in  order  that  its 
reaction   to   every  change  in   environment  may  be  followed.     The 

-  MacDougal,  D.  T.    Growth  in  Trees.    Pub.  307,  Carnegie  Inst.  Washington,  1921. 


172  BULLETIN    1059,    U.    S.   DEPARTMENT   OF    AGRICULTURE. 

writers  know  of  qo  single  method  or  basic  method  of  following  these 
reactions  which  promises  more  than  the  method  of  following  the 
plant's  condition  by  frequent  observations  on  the  osmotic  pressure 
or  sap  density.  In  the  case  of  trees,  the  termini  should  of  course  be 
studied,  but  it  is  probable  that  valuable  supplemental  data  can  be 
obtained  at  one  or  two  points  along  the  stem  and  on  side  branches. 
Through  this  method,  with  any  individual  tree,  it  may  be  possible  to 
depict  the  sudden  influx  of  sap  in  the  spring,  which  precedes  the  first 
growth;  the  gradual  increase  in  carbohydrates   as   the   new   tissues 

ntinue  to  function,  and  temporary  changes  due  to  water  supply 
and  water  loss.  Possibly  the  end  point  of  the  season's  photo-synthet  i<- 
activity  may  be  found,  if  there  is  any;  the  same  method  will  show 
the  changes  which  the  tree  undergoes  through  the  winter.  In  addi- 
tion to  osmotic  pressures,  the  starch  content  of  leaves  should  he 
examined  from  time  to  time.  It  is  self-evident  that  these  data  can 
only  he  interpreted  when  correlated  with  observations  on  both  the 
soil  and  atmospheric  conditions. 

A  method  which  would  show  the  rate  at  which  the  tree  i-  being 
supplied  with  water  would  be  a  valuable  adjunct  to  the  above;  but 
no  attempt  of  this  nature  is  known,  beyond  transpiration  studies.  It 
is  possible,  however,  that  a  means  may  be  devised  by  which  this  rate 
may  be  directly  determined.  There  is  room  for  a  great  deal  of 
development  in  these  lines. 

FIELD   OBSERVATIONS,  PHOTOGRAPHS,  AND   MAPS.  I 

The  "Daily  Observations'  form  in  the  series  for  climatological 
studies,  intended  for  field  observations,  is  thought  to  he  adapted  to 
universal  use.     It  is  of  a  size  to  fit  Forest  Service  loose-leaf  notebooks, 

6J  by  4  inches.  It  contains  lines  for  all  factor-  on  which  regular 
observations  are  likely  to  be  taken.  It  has  five  vertical  columi 
which  may  be  used  for  five  stations  visited  consecutively  on  the  same 
day.  or  for  a  single  station  visited  for  five  consecutive  observations. 
In  the  first  instance  the  numbers  of  the  stations  would  be  -tamped  at 
the  heads  of  the  several  columns  and  the  date  would  he  -tamped  at 
tli'1  bottom.  In  the  second  instance  the  day-  of  the  month  would  he 
-tamped  at  the  heads  of  the  several  columns  and  the  month  and  year 
at  the  foot  of  the  form.  In  either  case  the  exact  time  of  observation 
should  he  entered  in  the  second  line  of  each  column.  Since  observa- 
tions at  completely  equipped  stations  may  take  from  lo  to  20  minutes, 
the  time  entered  for  such  stations  should  be,  as  nearly  as  possible,  I 
time  of  reading  current  temperature  and  anemometer,  tin  -mil- 
lions hem-  subject  to  considerable  changes  in  a  few  minutes. 

Each  point  chosen  as  a  site  for  ecological  study  should,  in  addition 
to  a  complete  description  23  of  the  features  which  may  influence  -oil 


23 


The  "Description"  form  is  suggestive. 


RESEARCH   METHODS  IX  STUDY  OF  FOREST  ENVIRONMENT.      173 

quality,  insolation,  wind  exposure,  etc.,  be  witnessed  by  photographs 
and  topographic  map.  The  former  should  be  taken  from  as  many 
positions  as  possible  to  show  the  general  position  of  the  station  with 
respecl  to  topographic  features,  to  show  the  position  of  the  trees  which 
may  influence  atmospheric  factors,  to  furnish  a  map  of  the  canopy 
(by  vertical  view)  which  affects  the  insolation  at  the  ground,  and 
finally  to  show  in  detail  the  nature  of  the  ground  and  ground  cover. 
The  view  of  the  canopy  should  be  taken  as  nearly  as  possible  from  the 
position  which  will  be  occupied  by  solar  apparatus  and  evaporimeter. 
The  local  topographic  map  should  be  made  on  a  scale  adequate  to 
show  in  detail  the  immediate  surroundings  and  those  which  are  close 
enough  to  have  appreciable  effect  on  insolation,  wind  velocity,  etc. 
Especially  when  two  or  more  stations  in  close  proximity,  but  different 
presumably  in  some  essential  aspect,  are  being  studied,  should  the 
map  he  made  full  enough  to  bring  out  clearly  the  contrast  which 
exists  in  conditions.  In  such  cases  a  joint  map  for  the  several  stations 
may  well  he  used.  Such  maps  should,  if  possible,  be  restricted  to  the 
size  of  the  tabulating  Conns  used. 

Form  1. 

[('.  S.  Forest  Service,  Physical  survey.] 

DESCRIPTION    OF    OBSERVATION    POINT.    LIMITED    AREA,    TREE     ENVIRONMENT. 

Project  — — — -. 

Station  No Lrea  involved 

Forest  type Absolute  elevation 

Permanenl  sample  pint  No Bloek  No 

National  Forest Nearest  post  office 

County State 

Legal  location Exact  location 


PHOTOGRAPHIC    FEATURES. 

Aspect Gradient 

Distance  from  ridge Distance  from  channel 

Elevation  above  water Wind  exposure 

Air  drainage  condition 

Elevation  of  horizon  to  east ;  west ;  north ;  south 

<  haraeter  of  topography 


FOREST    ENVIRONMENT. 

Distance  of  nearest  trees  to  north ;  east ;  south ;  west 

Age  of  trees Height 

Diameter - Trees  per  acre 

Stand,  feet  b.  m Stand,  cubic  feet 

( irowtli  rate;  height,  diameter,  or  cubic  feet  per  A.  A 

Shading,  amount From  trees  on 

Light  at  the  ground,  per  cent Determined  by 


174       bulletin  1059,  u.  s.  department  of  agriculture. 

Summary  of  foresl  conditions  affecting  reproduction  or  the  special  tree  under  obser- 
vation 3hading,  exhaustion  of  soil  moisture,  influence  of  herbaceous  vegetation, 
underbrush,  etc 


ription  of  the  special  tree  under  observation  as  to  size,  growth,  vigor,  and  repro- 
ductive capacity 

SOIL    CHARACTER    AND    MOISTURE    SUPPLY. 


Depth  of  litter Character 

Depth  of  humus Present  moisture 

Penetration  of  humus  in  mineral  soil  as  shown  by  color,  inches 

<  lharacter  of  subsoil,  distinguished  from  soil 

Total  depth  of  soil  permeable  to  roots,  inches 

A  mount  and  kind  of  rock  fragments  in  soil 

<  lharacter  of  impermeable  substratum 

Soil  origin  (in  situ,  colluvial,  alluvial,  or  eolian,  and  from  what  rocks) 


I  i  imposition  and  class 

Drainasre Present  moisture. 


REFERENCES. 

Maps 

Reports 

Photographs 

Environmental  conditions  measured 

Moisture  samples Samples  for  anal)  sis 

i  I  analyses  filed 

Observations  by (date) 


APPENDICES. 

APPENDIX    A. 

VAPOR    PRESSURE    TABLES— WAGON    WHEEL    GAP,  COLORADO. 

The  accompanying  table  for  obtaining  vapor  pressures  from  reading  of  the  dry  and 
wet  bulb  thermometers  was  computed  by  Mr.  B.  C.  Kadel  for  a  pressure  of  21:42  inches, 
from  the  Ferrel  psychrometric  formula  used  by  the  Weather  Bureau: 


/       £/_  39  \ 
e=e/—0.000ZQ7B(t—t/)   \}+Y&Jl) 


in  which  t  and  tf  are  the  temperatures  of  the  dry  and  wet  bulb  thermometers  in  degrees 
Fahrenheit,  B  is  the  barometric  pressure  in  inches,  e'  is  the  saturation  pressure  of 
aqueous  vapor  at  the  temperature  //  of  the  wet  bulb,  and  e  is  the  vapor  pressure  cor- 
responding to  the  thermometer  readings.  The  constants  of  the  formula  were  deter- 
mined by  Profs.  Ilazen  and   Marvin,  of  the  United  States  Weather  Bureau. 


( 


tf-Si  i 


Table   8.—  Values  of  0.000367   B  \lJrY57l)  when  B=-}-"-   inches.     (Log.  0.000367 

B=7. 89549.) 


(' 

f— 32 
1+  1,571 

log. 

+7.89549 

Product. 

0. 95736 

9.98107 

7. 87656 

0.  007526 

-30 

0.  96054 

9. 98252 

7. 87801 

0.  007551 

-25 

0. 96372 

9. 98395 

7. 87944 

0.  007576 

-20 

0. 96690 

9.98538 

7.sM).s7 

0.  007601 

-15 

0.97008 

9. 98681 

7. 88230 

0. 007626 

-10 

0. 97327 

9. 98824 

7.88373 

0. 0076-51 

— 0 

0. 97645 

9. 98965 

7.  88514 

0.  007676 

Zero 

0. 97963 

9.  99106 

7. 88655 

0  007701 

0 

0. 98282 

9.  99248 

7. 88797 

0.  007726 

10 

0. 98600 

9. 99388 

7.  88937 

0.  007751 

15 

0. 98917 

9. 99527 

7. 89076 

0.00776 

20 

0. 99226 

9.  99667 

7.89216 

0.  007801 

25 

0. 99554 

9. 99806 

7.  89355 

0.  007826 

30 

0. 99873 

9. 99945 

7. 89494 

0.  007851 

35 

1.00191 

0.00083 

7.  89632 

0. 007876 

40 

1.00509 

0.  00221 

7. 89770 

0. 007901 

45 

1.00827 

0.  00358 

7. 89907 

0.  007926 

50 

1.01146 

0.  00495 

7.90044 

0.  007951 

55 

1.01464 

0. 00632 

7. 90181 

0.  007976 

60 

1.01782 

0.  00767 

7.9031O 

0. 008001 

65 

1.02101 

0.00903 

7. 90452 

0.  008026: 

70 

1.02418 

0.  01038 

7. 90587 

0. 008051 

7.5 

1.02737 

0.01173 

7. 90722 

0.  008076 

80 

1.  03055 

0.  01307 

7. 90850. 

0,  008101 

85 

1.03373 

0.  01441 

7. 90990 

0.  008126 

90 

1.03692 

0.  01575 

7.91124 

0.  008152 

175, 


176  BULLETIN    1059,   U.    S.   DEPARTMENT   OF   AGRICULTURE. 

Table  9-    Vapor  pressure,inches;  baro  neter  UM  inches:  depression  oj  wet  buTb(t.-tf)i°F. 


Wet  Bulbs- 16°  to-8° 


bulb. 

0.0 

-16  0 

0.01.5'.) 

-15  9 

.0160 

.0160 

-15.7 

.0161 

-15.6 

.0162 

-15.5 

.  0163 

15  1 

.01  til 

.0165 

15.2 

.0166 

•  15  1 

.6167 

-1.5.0 

.0168 

-11  9 

.0169 

-14  8 

.0170 

-14.7 

.0171 

-14  6 

.0172 

-14.5 

.0173 

-14.  1 

.0171 

-14.3 

.  017.5 

-14.2 

.0176 

-14.  I 

.0177 

-14.0 

.0178 

-13.9 

.0179 



.  01S0 

-13.7 

.  0181 

-13.6 

.0182 

-13.5 

.01 

-13.4 

.0184 

.01 

-13.1. 
-13.0. 


0186 
,0187 

Diss 


12.9 .0189 

. 0190 

.0191 

0192 

12  5 .0193 

.0194 

. 0195 

-  .0196 

12.1 .0197 

-12.0 0199 


1.0 


0.0083 
.0084 

.0084 
.00^.5 
.00-6 
.  0087 
.0088 
.0089 
.0090 
.0091 
.0092 

.  0093 
.0094 
.0095 
.0096 
.0097 
.0098 
.0099 
.0100 
.0101 
.0102 

.  0103 
.0104 
.0105 
.0106 
.0107 
.0108 
.0109 
.0110 
.0111 
.0112 

.0113 
.0111 
.011-5 
.0116 
.0117 
.Oils 
.011!* 
.0120 
.0121 
.  012-3 


2.0 


0  0007 
.0008 
.0008 
.0009 
.0010 
.0011 
.0011 
.0012 
.  0013 
.0014 
.  0015 

.0016 

.0017 

.001S 

.0019 

.0020 

.0021 

.0022  ! 

.0023  ! 

.0024 

.0025 

.0026 
.0027 
.0028 
.0029 

.0030 

.0031 
.  0032 
.  0033 
.  0034 

.  0035 

.  0036 
.0037 

.  0038 
.  0039 
.0040 
.0041 
.0042 
.  0043 
.0044 
.0046 


Wet  bnlb.       0.0 


-12.0      ..  0.0199 

-11.9 .0200 

-11.8 -0201 

-11.7 0202 

-11.6 -2003 

-11.5 .0204 

-11.4 .0206 

-11.3 .0207 

11.2 .0208 

-11.1 .0209 

-11.0 .0210 


-10.9. 
-10.8. 
10.7 
-10  6. 
-105. 
-10.  I. 
•10.3. 
-10.2. 
•10.1. 
•10.0. 


.0212 
.  0213 
.0214 
.021.5 
.0216 
.0217 
.0215 
.0220 
.0221 
.0222 


-99 .0223 

-9.8 .0224 

-9.7 -0226 

-9  6 .0277 

-9.5 .0228 

-9.4 .0229 

-9.3 .0230 

-9.2 .0232 

-9.1 

-9.0 .0231 


.0236 
.0237 

.hi-:  is 

.  0239 
.0210 
.0212 
.0213 
.0244 
.  0246 
-S.0 .02)7 


-8.9.. 
-8.8.. 
-8.7 
-8.6.. 

-8  •"... 
-8.  !.. 

8.2.. 
-8.  L. 


1.0 


0  0123 
.0124 
.0125 
.0126 
.0127 
.0128 
.0130 
.0131 
.0132 
.01.33 
.0134 

.  0136 
.0137 
.  0138 
.0139 
.0140 
.0141 
.0142 
.0143 
.0144 
.  014.5 

.  0146 
.0117 
.0149 
.  01.50 
.0151 
.  01.52 
.01.53 
.0154 
.  01.55 
.01.57 

.0159 
.0160 
.Mini 
.0162 
.  0163 
.0165 
.0166 
.0167 
.0169 
.  0170 


2.0 


0  0046 

.  0047 
.0048 
.  0049 
.0050 
.0051 
.  0053 
.0054 
.  1 1655 
.C056 
.0057 

.  00.59 
.  0060 
0061 
.0062 
.0064 

.0066 
.0067 
.0068 
.0069 

.11070 
.0071 
.0073 
.0074 

.11075 

.0076 
.0077 
.0079 
.0080 
.0081 

.0083 
.0084 

.OOs.5 

.0086 
.0087 
.0089 
.0090 
.0091 
.0093 

.OOOI 


.3.0 


Proportional  parts. 


0.0004 

.0006 
.0007 
.0008 
.0009 
.ooio 
.0012 
.0013 
.0014 
.0016 
.0017 


Degrees. 

0.  l 
.2 
.3 

.  1 
.5 
.6 
.7 
.8 
.9 


Inchfs. 
0.0008 
.0015 
.0023 
.0031 
.0O3S 
.0046 
.  00-53 
.0061 
.0069 


Wet  Bulb  -8°  to  -0°. 


Wet  bulb. 


0.0 


S.0 0.0247 

7.0 0248 

7.8 .0210 

7.7 .0250 

7.6 .0252 

-7.5 .0251 

7.1 .0255 

.0256 

-7.2 .0257 

-7.1 .0258 

.0260 

.0262 

.02C3 

.0264 

.0266 

.1 

.( 

.0270 

.0272 

027  1 

0275 


1.0 


0.0170 
.0171 
.0172 
.0173 
.0175 
.0177 
.0178 
.0179 
.0180 
.0181 
.0133 

.0185 
.0186 

.01s? 

.0189 

.0101 
.0102 
.0103 
.0105 
.0197 
.0198 


2.0 


3.0 


0.0094 
..0095 
.0096 
.0097 
.0099 
.0101 
.  0102 
.  0103 
.0104 
.  0105 
.0107 


0.0017 
.0018 
.0010 
.  0020 
.0022 
.0024 
.  002.5 
.0026 
.0027 
.  002,8 
,00.30 


.0109 

.0110 

.0111 

.0113 

.0036 

.6115 

.0037 

.0116 

.0038 

.0117 

.0039 

.0119 

.0041 

.0121 

.0013 

.0122 

.0041 

Wei  bulb. 


l.o. 

3.9. 
-3.8. 
-3.7. 
-3  6. 
-3.5. 


-3.4. 
-3.3. 

-3.2. 
-3.  1. 
-3.0. 

-2.9. 

2.8. 

2.7. 
-2.6. 
-2.5. 

2.  1. 
-2.3. 
-2.2. 

2.  I  . 
-2.0. 


0322 
0324 


0.0 

1.0 

2.0 

3.0 

0.0 

0.02 

0.0153 

0.0077 

.6309 

.  0232 

.0155 

.0079 

.0311 

.05 

.0157 

.0081 

.0313 

.02 

.0159 

.0083 

.0315 

.02 

H161 

.0084 

.03 

.02 

.0102 

.0085 

.0318 

.  02 1 1 

.0164 

.0087 

.0.320 

.0 

.0166 

.0089 

4.0 


.0215 
.02  47 


.  6325 

.02»s 

.0 

.  6320 

.  i  (252 

.63 

.02 

.0 

.0 

.0334 

.  0257 

.  0330 

.0259 

.  0338 

0261 

.0340 

.02 

.0342 

.020.". 

.0344 

.0168 

.0170 
.((171 

.0173 
.017.". 
.1(170 
.0178 
.0180 
.0182 
.0184 
.0186 
.diss 
.0190 


.0091 
.0093 
.0094 

.0096 

.0099 

.0101 
0103 

.0105 

.0107 
.one* 
.0111 


O.IMHKl 
.(KM  (2 

0004 
.0006 

.0008 
.0009 

.0011 

.0013 
.0015 
.0017 
.0018 


,0020 
,  I  h  122 
.0023 
.0024 

,0032 


RESEARCH  METHODS  IN  STUDY  OF  FOREST  ENVIRONMENT.      177 

Table    9. —  Vapor   pressure,    inches;    barometer   2142    inches;   depression  of  wet  bulb 

(t.-f),  °F—  Continued. 

Wet  Bulb  -8°  to  -0°— Continued. 


Wet  bull). 


0.0 


1.0 


-5.9 0.0276 

-5.8 .0278 

-5.7 .0280 

-5.6 0281 

-5.5 .0282 

-5.4 .0284 

-5.3 0286 

-5.2 .0287 

-5.1 0289 

-5.0 .0291 

-4.9 0292 

-4.8 0294 

-4.7 0296 

-4.6 .0297 

-4.5 0299 

-4.4 6301 

-4.3 0302 

-4.2 0304 

4.1 0306 

-4.0 (13117 


U 


0199 
.0201 
.  0203 
.0204 
.0205 
.0207 
.0209 
.0211 
.0213 
.0214 

.0215 

.0217 
.0219 
.0220 
.  0222 
.0224 
.  6225 
.0227 
.0229 
.0230 


2.0 

3.0 

).  0123 

0.0046 

.  0125 

.0048 

.0126 

.0050 

.0127 

.  0051 

.0128 

.0052 

.0130 

.0054 

.0132 

.0056 

.  0133 

.  0057 

.  0135 

.0059 

.0137 

.0061 

.0138 

.0062 

.0140 

.0064 

.0142 

.0066 

.01 43 

.  0067 

.  0145 

.0069 

.0147 

.007! 

.0148 

.  0072 

.  0150 

.  007 1 

.0152 

.0076 

.  0153 

.0077 

Wet  bull). 


0.0 


-1.9 0.0346 

-1.8 0347 

-1.7 0349 

-1.6 .0351 

-1.5 0353 

-1.4 0355 

-1.3 0357 

-1.2 .0359 

-1.1 0361 

-1.0 0363 

-0.9 0365 

-0.8 .0367 

-0.7 .0369 

-0.6 .0371 

-0.5 0373 


1.0 


-0.4. 
-0.3. 
-0.2. 
-Q.1. 
-0.0. 


0375 
3777 
0379 
0381 
0383 


0.  0269 
.0270 
.0272 
.0274 
.0276 
.0278 
.0280 
.0282 
.02S4 
.0286 

.0288 
.0290 
.0292 
.0294 
.0296 
.0298 
.0300 
.0302 
.0304 
.0306 


2.0 


3.0 


0.  0192 
.  0193 
.  0195 
.0197 
.0199 
.0201 
.0203 
.0205 
.  0207 
.0209 

.0211 
.0213 
.0215 
.0218 
.0220 
.0222 
.0224 
.  0226 
.022S 
.0230 


0.0115 
.0116 
.0118 
.0120 
.0122 
.0124 
.0126 
.0128 
.0130 
.0132 

.0134 
.0136 
.013S 
.0140 
.0142 
.0144 
.0146 
.0148 
.0150 
.0152 


4.0 


0.  0038 
.0039 
.0041 
.0043 
.  0045 
.  0047 
.0049 
.0051 
.  0053 
.0055 

.0057 
.  0059 
.0061 
.  0063 
.  0065 
.0067 
.0069 
.0071 
.0073 
.0075 


Wet  Bulb  0°  to  4= 


Wet  bulb. 


0.0 


0.0. 
0.1. 
0.2. 
0.3. 
0.4. 
0.5. 
0.6. 
0.7. 
0.8. 
0.9. 
1.0. 

1.1. 
1.2. 
1.3. 
1.4. 
1.5. 
1.6. 
1.7. 
1.8. 
1.9. 
2.0. 

2.1. 
2.2. 
2.3. 
2.4. 
2.5. 
2.6. 
2.7. 
2.8. 
2.9. 
3.0. 


3.1. 
3.2. 
3.3. 
3.4. 
3.5. 
3.6. 
3.7. 
3.8. 
3.9. 
4.0. 


1.0 


0.  0383 
.0385 
.0387 
.  0389 
.  0391 
.0392 
.0394 
.0396 
.0398 
,0400 
,  0403 

.0405 
.0407 
.0409 
.0411 
.0413 
.  0415 
.0417 
.0419 
.0421 
.0423 

.0426 
.0428 
.0430 
.  0431 
.0434 
.  0436 
.  0438 
.0440 
.0442 
.0444  | 

.0446 
.0449 
.0451 
.  0453 
.0456 
.  0458 
.0460 
.0462 
.0464 
.0467 


0.0306 

.0308 
.0310 
.  0312 
.0314 
.0316 
.0318 
.0320 
.0322 
.  0324 
.  0326 

.  0328 
.0330 
.  0332 
.0334 
.  0336 
.  0338 
.0340 
.0342 
.0344 
.0346 

.  0348 
.  0351 
.  0352 
.0354 
.0356 
.  0359 
.  0361 
.0363 
.  0365 
.0367 

.  0369 
.0372 
.0374 
.0376 
.0379 
.03S1 
.0383 
.0385 
.0387 
.0390 


2.0 


3.0 


0.0229 
.0231 

.  0233 
.  0235 

.0237 
.02311 
.0241 
.  0243 
.  0245 
.0247 
.0249 

.  0251 
.0253 
.  0255 
.0257 
.0259 
.0261 
.0263 
.0265 
.0267 
.0269 

.0272 
.0274 
.0276 
.0277 
.0280 
.0282 
.0284 
.0286 
.0288 
.0290 

.0292 
.0295 
.0297 
.0299 
.0302 
.0304 
.0306 
.0308 
.0310 
.0313 


0.0152 
.0151 
.01.56 
.0158 
.0160 
.0162 
.0164 
.0166 
.  016s 
.0170 
.0172 


.0176 
.0178 
.  0180 
.0182 
.nisi 
.0186 
.0188 
.0190 
.0191 

.0194 
.0196 
.0198 
.0199 
.0202 
.0204 
.0206 
.0208 
.0210 
.0212 

.0214 
.0217 
.0219 
.0221 
.0224 
.0226 
.0228 
.0230 
.0232 
.  0235 


4.0 


5.0 


0.  0095 

.Guyts  ■ 

.0098  i 

.0100  i 

.0102 

.0104 

.0106 

.0108 

.0110 

.0112 

.0114 

.0116 
.0119 
.0120 
.0122 
.0124 
.0127 
.0129 
.0131 
.  0133 
.  0135 

.  0137 
.0140 
.0142 
.0144 
.0147 
.0149 
.0151 
.0153 
.0155 
.  0158 


6.0 


Proportional  parts. 


0. 0037 

.0040 
.0042 
.0044 
.0045 
.0048 
.00.50 
.0052 
.0054 
.0056 
.  0058 

.0060 
.0063 
.0065 
.0067 
.0070 
.0072 
.0074 
.0076 
.0078 
.0081 


Degrees. 

Inches. 

0.1 

0.0008 

.2 

.0015 

.3 

.0023 

.4 

.0031 

.5 

.0039 

.6 

.0046 

.7 

.0054 

.8 

.0061 

.9 

.0069 

0.0004 


H  >  H  '.3— 22— Bull.  1059 12 


178  BULLETIN   1059,   U.    S.    DEPARTMENT    OF    AGRICULTURE. 

Table  9 Vapor    pressure,    inches;  barometer   21 M   inches;  depression    of   wet    bulb 

(t.-tf),  ° F.— Continued. 

Wet  Bulb  4°  to  8°. 


bulb. 


0.0 


l.d 0.0467 

(.  1        0470 

L2    0472 


1.  1. 


,0474 
.0476 
,047s 
,0481 
.0484 
.0486 
.0488 
.0491 


5. 1 '  .  0493 

5.  2 0495 

5. 3 0498 

5. 4 0500 

0502 

5.6 0505 

5.  7 0508 

0510 

0512 

6.0 0515 


6.  1 . . 
6.  2 .  . 

6.  t .  . 

6.  5 .  . 

6.7.. 

6.9.. 
7.0. . 

7.  1  .  . 
7.  2 .  . 
7.  3  .  . 
7.4.. 
7.  5 .  . 
7.  6 .  . 
7.7.. 
7.8.. 
7.  9 .  . 
8.0.. 


.  0518 
.  0520 
.0523 
.  0526 
.  0528 
.  0531 
.  0534 
.  0536 
.  0539 
.0542 


1.0 


0. 0390 
.  0393 
.  0395 
.  0397 
.  0399 
.0401 
.0404 
0407 
.  0409 
.0411 
.0414 

.0416 

.  0418 
.0421 
.  0423 
.0425 
.0428 
.  0431 
.  0433 
.0435 
.  0438 

.0441 

0143 

0446 

0449 

0451 

0454 

0457 

,  0459 

,0462 

.  0465 


2.0 


3.0 


4.0 


0. 0313 
.  0316 
.0318 
.  0320 
.0322 
.0324 
.0327 
.  0330 
.0332 
.  0334 
.  0336 

.  0338 
.  0340 
.  0343 
.0345 
.0347 
.  0350 
.  0353 
.  0355 
.  0357 
.  0360 

.  0363 
0305 
0368 
0371 
0373 
0376 
0379 
0381 
,0384 
.  0387 


.  0443 

.  0305 

.02ss 

.0211 

.0446 

.  0368 

.0201 

.  0214 

.0449 

.  0371 

.  0294 

.0217 

.0451 

.  0373 

.02% 

.  0219 

.04.54 

.  0376 

.0299 

.0222 

.  0457 

.  0379 

.0302 

.  0225 

.  0459 

.0381 

.0301 

.  0227 

.  0544 

.0467 

.0389 

.0.547 

.0470 

.0392 

.  0550 

.0473 

.  0395 

.  0553 

.0476 

.  0398 

.  0556 

.0479 

.0401 

.0558 

.0481 

.  0403 

.  0561 

.0484 

.0406 

.0564 

.  0487 

.0409 

.  0567 

.0490 

.0412 

.  0570 

.  0493 

.  0415 

0. 0235 
.  0238 
.0240 
.  0242 
.0244 
.  0246 
.0249 
.  0251 
.  0253 
.0256 
.  0259 

.0261 
.  0263 
.  0266 
.0268 
.  0270 
.  0273 
.0276 
.  0278 
.  0280 
.0283 

0286 
0288 

02!  II 

0294 

02% 
0299 
0302 
0301 
0307 
,  0310 

,0312 
,0315 
.03  is 
.0321 
.0321 
.0326  , 
.  0329 
.  0332 
.0335 
.  0338 


0.  0158 
.0161 
.  0163 
.  0165 
.0167 
.0169 
.0172 
.  0175 
.0177 
.0180 
.0182 

.0184 
.0186 

.0189 
.0190 
.0192 
.0195 
.0198 
.0200 
.0203 
.0206 

,0209 

0211 
0214 
0217 
0219 
0222 
0225 
0227 
0230 
0233 

,0236 

,0239 
,0242 

,0215 
0248 
,  0250 
.0253 
.0256 
.0250 
.0262 


5.0 


6.0 


,0081 
,0084 
,0086 

.OOSS 

.0090 
.0092 
.0095 

,OO0s 

,0100 
.  0103 
.0106 

.0108 

.0110 

.0113 

.0115 
.0117 
.0120 

.0123 

.oi  25 
.0126 

.012s 

.0131 
.0133 

.oo;;o 

.0139 
.0141 
.(lilt 
.0147 
.01.50 
.0153 
.0155 

.0157 

.0100 
.0103 
.0100 

.0169 

.0171 
.0171 
.0177 
.0180 
.0184 


0.0004 
.0007 
.0009 
.0011 
.0013 
.0015 
.0018 
.0021 
.0023 
.0026 
.0027 

.0029 

.0031 
.0034 
.  0036 

.0041 

.(Mill 
.0010 

.0048 
.005] 

.0054 
.0056 
.0059 
.0052 
.0064 

.0070 
.0072 
.0075 
.0078 


7.0 


Proportional 

parts. 

l)t  i/rees. 

Inches. 

o.  1 

0. 0008 

.2 

.0015 

.3 

.  0023 

.4 

.0031 

.  5 

.0039 

.6 

.0046 

.7 

.00.54 

.8 

.0062 

.9 

.0069 

,0080 

.0089  

,0092  

.0094  

.0097  

.0100  

.0103  

.o|o.i  0.0028 


Wet  Bulb  8°  to  ]2C 


Wei 
bulb. 


0.0 


8.0... 

s.l... 
8.2... 
8.3... 
s.t... 
8.5... 
8.6... 
8.7... 

9.0.. 

9.1.. 

9.2.. 
9.3.. 
9.4.. 
9.5.. 

9.7.. 

U.S.. 

10.0. 


0.0570 
.  0573 
.  0576 
.  0579 
.0582 
.0584 
.0587 
.  0590 
.0594 
.  0597 
.  0600 

.0603 
.  0606 
.0609 
.0612 
.0615 
.0618 
.0622 
.  0625 
.  0628 
.%31 


1.0 


0.  0492 
.  0495 
.  049S 
.  0501 
.0504 
.  0506 
.0509 
.0512 
.0514 
.0518 
.  0523 

.  0520 
.  0529 
.0532 
.0535 
.0538 
.0541 
.0515 
.05  is 
.0550 
.  0553 


2.0 


0.0415 
.0418 
.0421 
.  0424 
.0427 
.0429 
.  0432 
.  0435 
.0439 
.  0442 
.  0445 

.  0448 
.0151 
.0454 
.  0457 
.0461 
.0464 
.0468 
.0471 
.0474 
.  0470 


3.0 


0. 0338 
.  0341 
.0344 
.0347 
.0350 
.  0352 
.  0.355 
.0358 
.0362 
.  0365 
.  030S 

.0371 
.0371 
.  0377 
.  0380 
.0383 
.0386 
.0390 
.0393 
.  0396 
.  0398 


4.0 


5.0 


0.  0260 
.  0203 
.0266 
.  0269 
.0272 
.0274 
.0277 
.0280 
.0284 
.0287 
.0290 

.  0293 
.  0296 
.0299 
.  0302 
.0305 
.  0309 
.0313 
.0316 
.0318 
.  0321 


6.0 


7.0 


0.0183 
.0186 

.0189 
.0192 
.  0195 
.0197 
.0200 
.  0203 
.0207 
.0210 
.0213 

.0216 

.0219 
.0222 
.  0225 
.022s 
.0231 
.  0235 
.  023s 
.0210 
.  0243 


0.  0106 
.0109 
.0112 
.0115 

.0118 

.0120 
.0123 
.0120 
.  0130 

.0132 
.0135 

.013s 
.01  tl 
.0114 
.0117 
.0151 
.0154 
.0158 
.016] 
.0164 
.0166 


0.0028 
.0031 

.0034 
.  0037 
.0040 
.0043 
.0040 
.0049 
.0052 
.0055 
.  0058 


.0061 
.0064 

.0067 
.0070 

.007:: 
.0076 
.0080 

.mis:; 

.0086 
.0088 


8.0 


9.0 


Proportional 

pai 

Deg. 

Ins, 

0.1 

0. 0008 

.2 

.0016 

.0023 

.  1 

.  5 

.6 

.  0047 

.  i 

, .  |  r,  i 

.  OOB2 

.0070 

o.ooil 


RESEARCH  METHODS  IN  STUDY  OF  FOREST  ENVIRONMENT.       179 

Table  9. —  Vapor   pressure,    inches;   barometer  21.4%   inches;    depression    of    wei    bulb 

(t.-f),  °F.—  Continued. 

Wet  Bulb  8°  to  12°— Continued. 


Wet 

bulb. 


10. 
10. 


11 

11 
11 


10.3. 

10. 

10. 

10. 

10. 

10. 

10. 

11. 


11.  1. 
11.2. 
11.3. 
11.4. 
11 


II.!). 
12.0. 


0.0 

,0 

0.  0634 

0. 0556 

.  0638 

.0560 

.0641 

.  0563 

.  0644 

.0566 

.0648 

.  0570 

.  0651 

.  0573 

.  0654 

.0576 

.  0657 

.  0579 

.0661 

.  05s  5 

.  0665 

.0587 

.  0668 

.o.V.  in 

.0671 

.0593 

.0674 

.0596 

.O07s 

.0600  ! 

.  0682 

.  0604 

.0685 

.06117 

.  OOss 

.  0610 

.  0692 

.0614 

.0896 

.0618 

.0699 

.0621 

2.0 


0. 0479 
.0485 
.0485 
.0489 
.  0493 
.  0496 
.0499 
.  0502 
.  0506 
.051(1 

.0513 
.0516 
.0519 

.  0523 
.  0527 
.  0530 

.  05 ;:', 
.  0537 

.0511 
.0511 


3.0 


.  0405 
.0408 
.0411 
.0415 
.0418 
.  0421 
.0424 
.04:S 
.0432 

.  0435 
.04  58 
.0441 
.0115 
.0449 
.0453 
.0456 
.  0460 
.0453 
.0486 


4.0 

5.0 

6.0 

7.0 

8.0 

9.0 

0. 0324 

0.  0246 

0. 0169 

0.0091 

0.0014 

.0328 

.  02.30 

.  0173 

.  0095 

.0018 

.0331 

.  0253 

.0176 

.0098 

.0021 

.  0334 

.0256 

.0179 

.0101 

.0024 

.0338 

.0260 

.0183 

.0105 

.0027 

.0141 

.  0263 

.0186 

.0108 

.0030 

.  0344 

.0266 

.0189 

.0111 

.  0034 

.0347 

.0269 

.  0192 

.0114 

.  0038 

.0351 

.  0273 

.0196 

.0118 

.0041 

.0355 

.0277 

.  0200 

.0122 

.  0045 

.0358 

.0280 

.020.5 

.0125 

.  0048 

.0  561 

0283 

0206 

.0128 

.0051 

.9364 

0286 

.0210 

0131 

.0054 

.0368 

.0290 

.021  1 

.  0135 

.0058 

.0372 

.  0294 

.0217 

.  0139 

.  0032 

.0  575 

.  0297 

.  0220 

.0142 

.  0065 

.0378 

.  0300 

.  0223 

.  0145 

.  0068 

.0382 

.  0304 

.  0227 

.0149 

.0071 

.0386 

0308 

.0230 

.0153 

.  0074 

.0389 

.0311 

.  0233 

.0158 

.007  s 

6.  oooi 

Proportional 
parts. 


Wet  Bii.b  12°  to  16°. 


We1 
bulb. 


12.0.. 


12. 

12. 

12. 

12. 

12.5. 

12.6. 

12.7. 

12.  s. 

12.9. 
13.0. 

13.  1 . 
L3.2. 

13.3. 
13.4. 
13.5. 
13.6. 
13.7. 
13.x. 
13.9. 
14.0. 

14.  1. 
14.2. 
14.3. 
14.4. 
14.5. 
14.6. 
14.7. 
14.  s. 
14.9. 
15.0. 

15.1. 

15.2. 

15.3. 

15.4. 

15.5. 

15.6. 

15.7 

15.8. 

15.9. 

L6.0. 


0.0 

1.0 

2.0 

3.0 

0.0699 

0.0621 

0.0544 

0.0466 

.0702 

.0624 

.0547 

.0469 

.0706 

.0628 

.0551 

.047  5 

.0710 

.0632 

.0555 

.  0477 

.0713 

.0635 

.0558 

.0480 

.0716 

.0638 

.0561 

.04  S3 

.0720 

.0642 

.0565 

.0|s7 

.  072 1 

.0646 

.  0569 

.0491 

.07-S 

.0050 

.057  5 

.0495 

.  07  52 

.0654 

.  0577 

.  0499 

.  0735 

.0657 

.11.-,  SO 

.0502 

.  0738 

.  0660 

.0583 

.0505 

.  0742 

.0664 

.05s7 

.0509 

.0746 

.  0668 

.0591 

.0513 

.0750 

.0672 

.  0595 

.0517 

.  0754 

.0676 

.  0599 

.0521 

.  0757 

.01)79 

.0602 

.  0524 

.0761 

.0683 

.  0606 

.  052  8 

.0705 

.0687 

.0610 

.0532 

.0768 

.0690 

.0613 

.  0535 

.  0772 

.0691 

.0617 

.  0539 

.0776 

.0698 

.0021 

.  054  1 

.07  so 

.  0702 

.  0625 

.0547 

.  07S4 

.0706 

.0629 

.0551 

.  07S7 

.0709 

.0632 

.0554 

.  0790 

.0712 

.  0635 

.  0557 

.0794 

.0716 

.  0839 

.  0561 

.0798 

.  0720 

.0642 

.  0565 

.0802 

.  0724 

.0646 

.  0569 

.0800 

.0729 

.0050 

.  0573 

.  0810 

.  0732 

.  0654 

.  0577 

.0814 

.  0736 

.  0658 

.  0581 

.0818 

.0740 

.0662 

.0585 

.0822 

.  0744 

.0666 

.0589 

.0826 

.  0748 

.0670 

.  0593 

.0830 

.  0752 

.0674 

.0597 

.0834 

.0750 

.  0678 

.0601 

.0838 

.0760 

.0682 

.0605 

.  0842 

.0764 

.  0686 

.0609 

.0846 

.0768 

.  0690 

.0613 

.0850 

.0772 

.  0694 

.0617 

4.0 


5.0 


0.0389 

.0.92 
.  0395 

.  0399 
.  0402 

jil  15 

.0109 
.0413 
.0417 
.0421 
.0424 

.0427 
.04  51 
.0435 

.  04  19 
.  0443 
.044:1 
.  0450 
.0454 
.0457 
.0461 

.0465 

.0469 
.  047  5 
.  0476 
.0479 

.ills', 

.0487 
.0491 
.  0495 
.  0499 

.  0503 
.  0507 
.0511 
.  0515 
.  0519 
.0523 
.  0527 
.  0531 
.0535 
.  0539 


0.0311  0. 

.0514  . 

.0318  . 

.  0322  . 

.  0325  . 

.  0328  . 

. 0332  . 

.0330  . 

.0340  . 

.0341  . 

. 0347  . 

.0350 
.0  554 
.0358 
,0362 
,  0366 
,0369 
0372 
.0376 
,  0379 
.  0383 


,0387 
,0389 

.0395 
,0398 

,0401 
,0405 
.0409 
.  0413 
.0417  , 
.0421 

.0425 
.0429 
.0433 
.  0437 
.0441 
.0445 
.0449 
.  0453 
.0457 
.0461 


6.0 


0233 
0236 

0210 
0244 

(1217 
11250 
0254 
1125s 

,0262 
,  0206 
.0269 

,  0272 
.  0276 
,0280 
,0284 
,0288 
.0291 
.  0295 
.0299 
.  0302 
.  0306 

.  0310 
.0314 
.03  IS 
.0321 
.  0324 
.  0427 
.  0431 
.  0435 
.  0439 
.  0343 

.  0347 
.0351 
.  0:555 
.  0359 
.  0363 
.  0367 
.  0371 
.  0375 
.  0379 
.  0383 


7.0 

8.0 

9.0 

10.0 

0. 0156 

0. 0078 

0.0001 

.0159 

.0081 

.  0004 

.0163 

.0085 

.0008 

.0167 

.0089 

.0012 

.0170 

.  0092 

.0015 

1 

.0173 

.0095 

.0018 

.0176 

.01199 

.0021 

1 


.0180 

.  0103 

.0025 

.0184 

.0107 

.0029 

.diss 

.0111 

.  003  5 

.0191 

.0114 

.  0036 



.0194 

.0117 

.  0039 

.0198 

.0120 

.  0043 

1 

.0202 

.0124 

.  0047 

1 

.  0206 

.0128 

.  0051 

.0210 

.  0132 

.  0055 

.0213 

.  0135 

.  0058 

.0217 

.0139 

.0062 

.0221 

.0143 

.0066 

.0224 

.0147 

.0069 

.0228 

.0150 

.  0073 

.0232 

.0154 

.0077 

.0236 

.0158 

.0081 

.0240 

.0162 

.  0083 

.  0243 

.  0165 

0086 

1 

.  0246 

.  0168 

.0090 

.  0250 

.0172 

.0094 

.  02,54 

.0176 

.0098 



.  0258 

.01S0 

.0102 

.0262 

.0184 

.0106 

.0266 

.0188 

.0110 

0.  0034 

.0270 

.0192 

.0114 

.0038 

.0274 

.0196 

.0118 

.0042 

.0278 

.0200 

.0122 

.  0046 

.0282 

.0204 

.0126 

.  0050 

.  0285 

.0208 

.  0130 

.  0053 

.0289 

.0212 

.0134 

.0057 

.  0293 

.0216 

.  0138 

. .  0060 

.0297 

.0220 

.0142 

.0064 

.  0301 

.0224 

.0146 

.0068 

.  0305 

.0228 

.0150 

.01172 

Proportional 
parts. 


Ins. 
0.  0008 
.  0015 
.  0023 
.0031 
.  0039 
.0047 
.0054 
.  0062 
.0070 


If 

0 

9- 

.1 

.2 

.3 

.4 

.  0 

.6 

.  7 

.8 

.9 

180 


BULLETIN   105!),    U.    S.    DEPARTMENT   OF   AGRICULTURE. 


table  9 -Vapor  pressure,  inches;  barometer  21J,.i  inches;  depression  of  u    i 
LABLE  '  bUib    (t.     f),  °F.— Continued. 

Wet  Bulb  16°  to  20c. 


bulb. 


I 
17.0 


17.1 



17.4 

17..". 

17.6 

17.7 

L7.8 

17.9 

1S.0 


18.1.. 
18.2.. 

18.4.. 
18.5.. 
18.6.. 

In. 7.. 

18.8.. 
18.9. 

19.0.. 


19.1. 

19.3. 
19.4. 
19.5. 

19.7. 
18.8. 
19.9. 
20.0. 


0.0 


L.O 


.0854 
.0857 
.0862 
.0856 
.0870 
.0874 
.0878 
.  i  (882 
.0886 
.0891 

.  0895 
.0899 

.090', 
.II9U7 
.  0912 
.0916 
.1)920 
.  0925 
.0929 
.  0933 

.  0938 
.  0942 
.0946 
.0951 
.0956 
.0960 
.0964 
.0969 
.0974 
.0979 

.0984 

.09ss 
.0992 
.0998 
.1002 
.1007 
.1012 
.1017 
.1022 
.1026 


0.0772 
.0776 

."77'.! 
.0784 

07SS 
.  0792 
.0796 
.0800 
.0804 
.0808 
.  0813 

.0817 
.0821 
.  0825 
.0829 
.0834 
.0838 
.0842 
.0847 
.0851 
.0855 

.0860 

.0864 
.0868 
.  0873 
.0878 
.0882 
.OssO 
.0891 
.  0896 
.0901 

.0906 
.0910 
.0914 
.0920 
.0924 
.0929 
.  0934 
.  0939 
.0944 
.  0948 


2.0 


3.0 


0.  0694 
.0698 

0791 

.  0706 
.0710 
.0714 
.0718 
.0722 
.0726 
.  0730 
.  0735 

.  0739 
.0743 
.0747 
.  0751 
.  0756 
.0760 
.0764 
.0769 
.  0773 
.0777 

.  0782 
.0786 
.0790 
.  0795 
.0800 
.0804 
.0808 
.0813 
.0818 
.  0823 

.0828 
.0832 
.  0836 
.0842 
.0846 
.  0851 
.  0856 
.0861 
.0866 
.0870 


0.0617 

0621 

0625 

.0629 

0633 

.  0637 

.0641 

.  0645 

0649 

.  0853 

.  0657 

.  0661 
.0665 

.0669 
.  0673 
.0678 
.0682 
.0686 
.0691 
.  0695 
.0699 

.0704 
.0708 
.0712 
.0917 
.  0722 
.0726 
.  0730 
.0735 


4.0 


5.0 


0.  0539 

.054-; 

0546 
.  0551 

0555 
.  0559 
.  0563 
.  0567 
.0571 
.0575 
.  0580 

.  0584 
.  0588 

.0592 
.0596  I 
.0600 

0604 
.  0608 

0613 
.0617 
.0621 

.0626 

.0634 
.0639 
.0644 
.  0648 
.  0652 
.  0657 


).  0461 
.0465 

0468 
.0473 

0477 
.0481 
.0485 
.0489 
.0493 

0497 
.0502 

0506 

.0510 
.0514 
.051s 
.0522 
.0526 
.0530 

.0539 
.0543 

.0548 

.0552 
.0556 
.0561 
.  0566 
.  1 1571 1 
.0571 
.  0579 


6.0 


n 

0390 
.0395 

0399 
.049.; 
.  0407 
.  04 1 1 
.0415 
.0419 
.0124 

.0428 

.01  12 
.  0436 

.  0440 
.  0445 

.0119 

.0453 

.045s 

.0462 
.0466 

.0471 

.0  17.", 

.0179 

.0484 
.0488 

.0492 

.0  100 
.07)01 


7.0 
0.0305 

8.0 

9.0 

0.  0228 

0.017,0 

.0309 

.02  _ 

.0|7,| 

.0312 

.0236 

.0157 

0317 

.  0240 

.0162 

.0721 

.  02 1 1 

.ol  or, 

.0327, 

.0248 

.0169 

.03 

0252 

0173 

.  0.333 

.  0250 

.0177 

0337 

.0260 

.0181 

.0341 

.0264 

.0185 

.0 

.  0268 

.0190 

.0710 

.0662 

.  05s  i 

0506 

.  07 15 

.  0G67 

.07,  si) 

.0511 

.077-0 

.0072 

.07,0  1 

.0516  . 

.  0754 

.0676 

.0" 

0520 

.  075s 

.0680 

.0602 

.0524 

.0764 

.90s', 

.0608 

.0530 

.0768 

.0090 

.0612 

.0534 

.  077:; 

.  0695 

.0617 

.0539 

.0778 

.07(H) 

.  0622 

.0544 

.0783 

.0705 

.0027 

.05 

.0788 

.0710 

.06  - 

.  05 

.0792 

.0714 

.01 

558 

,  03.50 
,0354 
.0358 
,0362 
.0367 

.07,71 

0375 

.0  si 
.0388 

.0400 

.0  107, 
.0110 
.0111 
.OllS 

.0423 
.0428 
.0433 

.0442 
.0446 
.0452 
.0456 
.0461 

.0471 

.0480 


.0272 

.0280 
.0284 
.0289 

.0297 
.0302 
.0306 

.07,10 

.0314 
.0318 

.0345 
.0350 

.0402 


10.0 


0.0072 


0194 

01 9s 

,0202 

,0211 
.0215 

.0219 
.0224 

.0236 

.0240 
.0211 
.0249 
.0254 
.  0258 

.0207 
.0277 

.0300 

.0310 
.0315 


.0112 

.OIH, 
.0120 
.0124 
.012s 
.0133 
.0137 
.0141 
.0140 
.0150 
.017,1 

.0158 
.0162 
.0166 

.0171 
.0176 
.0180 
.0184 
.0189 
.0194 
.0199 

.0203 
.0207 

.0211 
.0217 
.  0221 
.0221', 
.0231 

.  1 12 1 1 
.2045 


RESEARCH  METHODS  IX  STUDY  OF  FOREST  ENVIRONMENT.       181 


<3 

■  ~    CO 

-<-»   — 

u 

Ph 


o 


.  00  CO  CO  — i  Ol  N  ^  M  C 
so  O  -  M  ?5  K  -t  '"  C  t^ 
SOQOQOOQOO 

,=  00000000  0 

1° 


■«  _; 

v.  O 


•    •    ■    •    •  —  o  —  t^cMoo--rOiLi- f- 

a  oiocHHNnsi'i' 

O  O  —    I   —    —    ^H    *—   —  —    — 

o oooooooooo 

1   !   !   !   '.   •   •   '■   ■   !   '•     ••;;:;;;:     •■•■•*••   ;^3 

CM  t^  CM  t^  0«»  1^- CM  t^  OI  t~  11     N  ?l  t-  ^  3C  M  OJ  ■*  Oi  i-O     O  CO  — I  CO  CM  l^  CM  00  Tf  Ci  •^a'OOtOINNMffiiO 

HiHWiNMCC-"-"'"'*®      et»NXK00100"     NNWM^^iOiflso©  NNKOIOIOOh-N 

OOOOOOOOOOO      000—000  —  —  —      ——————————  rt-J-_(rtNMNMN 

ooooooooooo  ocoooooooo  oooooooooo  oooooooooo 
d 


q 
co 


o 
cm 


O  lO  O  >1  O  UJ  O  Li  O  "3  O 

OOOOOOOOOOO 


oooo     OOOOOOOOOO 


X^O^OiCOCNN 
OiCOh  NfNrocc-*  -r 

OOOOOOOOOO 


c^r^cooo^OLi^Ht^oo 

*U50»NXX0100 
OOOOOOOOOO 


x  r:  /  ?:  z  ^  x  ^  x  ^  x 

CNNXXOJO'.OC 

1  —  —  —  —  II  "1  11  11 

OOOOOOOOOOO 


O 

d 


COOO  *C  O  '-1  —  CO  —  CO 

—  wncoNNZ  x  r.  ~. 
n  n  ri  n  ri  ri  ri  11  n  n  n 

OOOOOOOOOOO 


—  CO  —  t>-  CM  t^-  el  00  CO  Oi 

oo  —  —  <^^(^^coco'^,•^, 
cccocococ-ir^cococico 

o  oooodoooo 


loiocNsxxaao 
cocococococccococo  — 
OOOOOOOOOO 


OOTjiCiOr-SMXTfO 

i-f>     —J-     — -     »—      —       —     «^     v*H     __,      — 

OOOOOOOOOO 


q 


TfOi-fCi-fOi'+'O'i  r.  — 

-^  u  ~^  — •-  *^  -V  — ^  -^  prt  -^  -^ 

OOOOOOOOOOO 


~-3)OOiOrttO'JN  COXMOnnCOrtNIN     t^CMOCCOOLIOcOCMOO 

i-xxoioo  —  —  cmcm  «  ?;  •]•  3  '•■•  °  '■-  ^  N  °°    ' 

71**   T'T'   ^w^   ^^   **^*  ^^*   "**"   ^"*"   "™"   ""*"'  ^5^  ^^  *^*  "^"F  ^T*  ^^  ^T^  *^T*  ,9^  ^^ 

oooooooooo  ooooooo- 


XOlfflOOH  CM  CM  CO  CO 

•*  t»i  i*  us  io  i"  i.-;  i/:  io  "O 


l    —    —        T   ^   ^T"  ^T1  ^CP  TP  ^T*  ^P   ^  ^T"        ^T"  ^T  ^   "J  LJ  L^   LJ   »T7    LC3  ITJ 

OO  oooooooooo  oooooooooo 


q 


n  r-  n  r-  o<  r-  - 1  x  :-   /•    - 

O  O  —  —  n  11  c"  r?  —  —  >~. 

— *-        — • «       — *-       *^t       — *•       •*•       *-**       T*«       w       —        — 

OOOOOOOOOOO 


"X.   ^X^XMS'fO  >-1 

'Vc;i-n  /  x  ~  ao 

—  —  —  —  —  —  -t--t<^*iLi    li  li  n  i  -  1 1 

oooooooooo    ' 


rJtOHNMOOMa'fC 

OOoOoOoooo 


©nkioxcisiohh 

LO  lO  IC1  lO  L^  LO  1C  CO  O  CD 

oooooooooo 


5~  7i 

'  •>  9_ 

CO  o 

<w  cm 

:_>  pa 


PQ 

> 


O  >1  O  '1  O  '1  o  co  —  cc  — 


q 


X«ClfflOO'--MN« 
-t*  —  —  -r  i~.  >~  't  •'.  i~.  't  >. 

oooooooooo  o 


co  —  cc  —  O  —  l-  cN  1^  co 

r f  >C  >1  CC  to  r~  l~-  BO 

<"  •"  •"  IOU31Q  1Q1O1AU3 


oooooooooo 


O—  0'~  —  o—  t^cooo 

X  C5  35  O  —  —  O)  C^  CO  CO 

>0"0L0t0OO«COOO 

oooooooooo 


CO  oc  •*  O  '-0 


I  C  N  00  rj< 


-t<  -r  li  i.i  co  f^  t^  oo  oo  0 

OCO'OCOCCOOCOOCO 

oooooooooo 


q 

CO 


x  cr  x  -.-.  '  --  s  —  — .  —  r 
■  i  co  w  i  ~  i  -  x  y  r.  ~  o  o 

'1  >~  '"  •*  I"  l~  I"  I" 

oooo  o  o  o 


—  O.  —  ~   —  O  1 1  O  '  1  —  COrHCOCNX«X-tOCO  — it^COOOrfOLl  —  t^-CO 

■Mr-icoco»r,'iLoo  coNNXMaao-rn  cNtNccco^icincocON 

•o  •-:      -  o  -o  -r   _  -O  CO  CO  CO  CO  cooecoooM-N  t--  l~  l^-  t^  t^  t^-  1^  t~  b-  l*. 

^ooooooooo  oooooooooo  oooooooooo 


q 


SO '  1  '  1   —    —  I  -  I  -   X    X 

OOOOOOOOOOO 


CJNTI  /.  M  /.Tftarfa 
OCiOO*— i  —  CNOICOCO 

oooooooooo    oooooooooo 


irj  O  O  w  N  CN  N  M  O  •* 
*"C"O!OcONN0tjX  - 


OiLI  —  CC  N  X  M  a  LO  h 
OiO  —  —  CMHCOCO-*llO 
I  -  X    X     /    X    X    X   X   X.   X 

oooooooooo 


o 


-+iOi-rOi  —  O—  O'lO'l 
— i  —  0<  H  CO  CO  —  '  1  '1  EC   CO 

OOOOOOOOOOO 


o  •  i  o  o  —  co  c^  t^  in  cc    co  oo  co  o  li 

t^  I  -  X,  X  Ci  Oi  O  O  —  —     ' 

r—  r--f~i— r~(-  x  x  x  x 

oooooooooo 


_  li  —  cO<N 
CM  Ol  CO  CO  -#  'O  Li  CO  CO  t^ 

x  x  X  x!  x  x  x  x  x  x 

oooooooooo 


t^COOi-^OcC- t^cOCi 

h-OCOOOiOO-  — iO$tM 


ccxc/.xaaaai 
oooooooo< 


>Oi 

>o 


q 
co 


CNNMNC-INNXCC  X  CO 
OiOiOO  —  —  cMUcoco  — 
NNXOOXXXXX/  X 
OOOOOOOOOOO 


00  n  X  —  Oi  •>*<  O  li  o  CO 
—  n  li  co  co  r-  x  x  0  oi 

X    X    X    X   X   X   00  00  00  00 

oooooooooo 


NNtNX^aTtiOcOH 
OO  —  —  CNCMCO-P-fLI 
OiCiOiOiOiOiOiOiOiOi 
OOOOOOOOOO 


cOtNXCOOiCOcOtNN 

LOcocrsNKoaoo 

OCiOOiOOiOiOiOO 

oooooooo—— 


o 

(N 


OLIO'IO'IOCO—  CO  — 
t-  r^  X  X  Oi  Oi  O  O M 

x  x  x  x  x  x  0  a  a  a  a 

OOOOOOOOOOO 


CO  —  CO  OI  t-  CM  X  CO  00  -^f  OLIOCOCM^CMOO— Oi  -tOCn  f-COOO-^OO 

HCOCO  —  —  '1'IOCOI^  XOOCiOiOO—  —  CMCM  CO^-T'1'"cOcOt^0O00 

a  a  a  a  a  a  a  a  a  a  oaoaooooco  oooooooooo 

OOOOOOOOOO  OOO— 'i—  i—  — i  t-4  y— (i—  t— (i—i—  i—  t— i^-t— 'i—  i— »- 


OOcOOOCOXCOOO-^'Oi-^'Ci  ^fCiLlOLIOcO  —  COCM  XCOX^OLOOCONN  cMOO-^OiLI- COCM00-* 

—  oijoccNNXxaa  oo  —  "Mcmcoco-^-^li  occcosxxaaoo  —  —  uoico  —  -*»ilio 

aaaaaaaaa  aa  oooooooooo  oooooooo—<-h  —  —  —  —  —  — <  —  ,—  ^-<  ■?-< 

OOOOOOOOOOO >--->— I  — I  ^H  — <  ,—  —-  —  —  —  —  —  -—  ^HrH— H  i-H  i—  — •  — (— I-  — 

d 


2 


q  ' 
d 


COrJCOHCOrJCOMNCNN      CMt^CMOOC-rOOTtlOl'^O     CO"—  OCMXCOOO'^O'O     O  CON  NM  ffl^OCOIN 

2Ji3222;2;ii2SSs9t^L-    xccoioioo  —  —  cmco    co-'cf^LiLicocot^oooo    oiooo  —  —  cmcoco-* 

OOOOOOOOOOO     OOOO—  —  —  —  —  —     —  —  —  ——  —  —  —  —  —     —  —  CMCMCMCMCMCMCMCM 

d 


3 

IS 


d-^cMCO-*lOCOt^OOCi 


—  NCO^lflcCNOOOO 


—  cMCO-^rLICCI^CCOiO 


<cMCO-*lOCOt^00Oid 


ddoOOodddd—      —  —  —  —  —  —  —  —  —  CM      cMcicMCMCMC^cMCMCNCO     COCOCOCOCOCOCOCOCOTli 

CNNINcNcNtNcNNMMf)      CN  M  C)  N  CI  C)  N  N  fl  N      CNCMCNCMCMCMcMCNCNCM      CMCMcMCMcMCMcMcMcMcM 


182  BULLETIN    1059,    U.   S.   DEPARTMENT   OF   A(ii;l(  T l/l  I '  IM 


— 
- 


- 
: 


X 

*. 

- 

k 

5s 

< 

n 

"C    - 


- 


—       -  * 


-- 
■ — 


- 
- 
re 
pq 

i- 
- 


v. 


Proportional 
parts. 

X  e  —  —  re.  i  -  '-"  —  — 

-S  c  —  ci  tc  ?t  rf  '"  -  i^ 

5  = 

?  —  NM'f'OCONOC    J! 
~                                        ... 
t»  — 
- 
3 

16.0 

* 

■ 

'3 

lOCOCONOOOOOiOOH 

x  —  —  r-  co  re  [^  re  ~  re 

i-H  cm  co  cc  —  •-.  -e  e  i-  t- 

c  i  re.  e  c  i  r.  -e  c  i  r-  e  c  i 
x  x  —  e  —  —  ci  ci  re  — 

Cl  Cl  Cl  Cl  Cl  Cl  Cl 

o 

q 

; 

• 

X 

Cl 

— . 

- 

f  cocc  ca  »e  :i  /_  t  ; 

W*^i:.L-;ONt>  X    C 

coocrrrrccoo 

cc  cm  re.  ■  e  —  x  ■  e  —  x  >  ~. 
cro  c  re  —  ci  ci  re ■  r 

—  C^C^C^C^C^CMCMCNCM 

ci  re.  c  ci  re.  c  ci  re.  re  — 
co  <o  t~  <x  x  e.  —  —  —  c  i 
cm  ci  ci  c i  c i  ci  cc  re  re  cc 

- 

r-- 
■<* 

M  O!  i"  -  1-  K  3".  iC  ^  O 
lOiOcONNO&QOOOO 
e: n  CJ 

reCCCCrcrerereO 

iNOO^Ht^cooocoa 
—  —  C3  co  co  ■*  i  e  i  e  r^  "C 

C<1  CM  (M  CM  (M  C-l  Ci  Cl  Cl  '.  1 

OOOOOCCCCC 

i:  -i  "/.  -r  c  i  — "Ci-m 
i  -  x  X-  rr-  re  e  —  c  i  c  i  re 
c^c^cicirorccecccccc 

co  cc  re  e.  -e  re  re- 
re  —  ■  .  ■ '.  -e  i  -  i  - 
cc  cc  cc  re  c*  cc  cc 

e  re  re- 
x  e.  e- 

- 

CM 

ci 

c 

—  t-  CC  rr.  ' 7  —  l^^3)'5 

CC  CC M  lOCO^Ol--  t-  X 

CI  CI  CI  CI  CI  CI  CI  CI  CI  CI 

cccrrcrecrrcc 

HNCOOcONO>>OHN 

-.  T.  3  t-1  «-H  C4  Cl  cc  —  — 
CMCMCOCOCCCCCCCCCCCC 

cccccccccc 

re  re  e  ci  x  >e  ci  x  >c  — 
in  'C  iM-  x.  c  rr.  e  — 

rerercccccccccce 

rrrerercreecccc 

i  -  —  —  i  -  —  —  i- 
—  ci  re  re  —  •-.  •'. 
— 

e  e  e  e  ~  e  ~ 

•e  i  -  i  - 

—  —  — 

ci 

CO 

re 

c 

~.  >~  —  t-  cc  r~.  ■  e  —  t—  cc 
c  —  ci  ci  re  re  -~  lc  »c  e 

-  -  -  -  -  ~  -  -  ^-  - —  -~  p^  p*cj  re 

rr. '  e  —  x  -*  c;  t—  cc  :"•  ■ " 

CCt^XXCiOOr-^   Cl 

rercrercrc  —  —  —  —  — 

hn^Ocsk  e  e  rr  e 

rr  rr  —  ■  .  •  .    _  i  -  i  - 

©MOCOCOOCC 
_.  re  —  —  ci  re  re 

—  >~.  'C  '".  ir  •".  'C 

- 

CO 

9 

r—  cc  re  <~  —  t-  re  ce  'c  — 
x  ■:-  rr  c ci  ci  cc  -i- 

cccorc— f-r-*'*'''— ~  ~~  -~ 

—  '  e  i  :  ■_;  t  -  i  -  x  ss  s>  o 
OOOOOOOO'OC 

~  -e  re  re-  ■  -.  c  i  —  ■  -  c  i  x 

ci  ci  re  —  —  ie  e  cc 

ir  e  >r  •'.  ■  .  <r  '*.  >r  <r  'r 

—  ^_    /- y    — 

i  -  x   x   r   e  q  — 

r  i  r  i  -; 

10.(1 

o 

cc 

— 
C 

c 

cc  ci  x  -r  c  i-  —  r-  cc  rr. 
-i^i'/  *r.  — .  re  — <  - — ' 

r-  — '-c»C'C'C 

ooooooooco 

OHNiJIOCOWO!  'C  Cl 
CM  CO  cc  -r  ■  -  '  -  i  C  I  -  X. 
• '.-".':» ~  '  :  i  e  '  ~  '  ~  '  e  ■  . 

~~~~  —  ~  —  —  ~~ 

/ i~rcei~reree 

/  ~  re  Z.  —  c  i  c  i  re 

■  e  .  e  re  e    z  e  e  e  e  cc 
eeeeeeec  ;  : 

r  i  r.  e  c  i  r-  e  c  i 

_  ■  -  i  -  /   e 

-    _ 

e  fj  r- 

i~ 

e 

X 

CC 
lie 

re 

-;cm  x  —  c  co  ci  x 
*mio(C!ON  x  ^.  rr-  rr. 
"~  '  c  >c  ic  »r  >c  >~  »c  ie  't 

-C  ;:c  c.  e  :i  /  -  c 
~ ci  •.  i  re  —  —  ■  .    - 

'   i  ~~  =  i  ■=  i  =  = 

■e  ci  —  '"  —         3r- 

e  i-  i-  x  re-  re.  -_ ci 

e  e  cc  cc  t—  t»  t-  r- 

—  /'-  —  /•-.— 

■-  i  - 
_-  ■_-  h-  i_-  t- 

x  •-  — 

- 

le  t  ■»  t- 

v  0 

cc  ?■/-;  /-  ;-:i  cc 

—  CJ  N  CC  ~  ~~  '"■  —  ~'z  t"" 

M  x  -r  —  i  -  rr  C  vr  ci  ~ 
x.  x  rr.  c  c  —  ci  ci  re  re 

vc  cc  cc  i  ^  i  -  i^  i  -  t ~  i  -  i  - 

oocccccccc 

•  e—  x  —  e  i  -  —  r 

—  i  -  ■  .  -e  i  -  i  - 

t-  t-  ij.  i_~  i_-  !_;.  t-  i_r  cer  i. 

e.  cc  ■■-.  s.  e  re  re. 

e  —  r  i  .  i  :  r. 

L  '£ 

/  7  / 

t> 

e 
C 

C  CC  CI  X  -!-  C  C  CI  S  -r 

c  ~  ■ ci  re  re 'C 

t^  t~-  I~  l~  1-  I^  I-  t-  >~  l~ 

rr  co  ci  os  lOHx^ots 

•-r;  rr;  i  -  i  -  x  rr.  ~  C 

i-  r-  i^  i~  i~  t-  \-  /    /   s 

oooocooc 

e  ci  x  ie  ci  / :  . -  — 

ci  ci  rr  —  —  ic  c  ei-  x 

/    y 

re  re  e  re  e  r    - 

!- 1- 1 X 

:    rr.  —  r  i  r  i  :  r 

x    ^    -  r.  r                 r.  ;. 

-  re  e  e 

6.0 

:  r.  i  -  —  i  -  -•-  x  -t-  re  —  ~  i 
i--  t-  x  rr.  ~  c  re  —  ci  ci  re 
t^t~t^lr~t~ooooocpoc 

rrcrecreccreccc 

x  -t-ci-rercicci  x  .- 
co  Tf  ■  .  ■  e  :.r  _  i  -  /  x  _ . 

£    X  X  X   y     /■ 

ccccccccrrc 

—  t e 

re  e  —  ci  ci  re 

re-  re.  e-  re.  re  e  e   - .  zr.  e 

:     - 
e  i  -  >    >>   e.  e  r_ 

.--  _  e 

re  re  i  - 

—  r  i  zj 

i " 

X 

c 

t^.  CC  S-  '"  —  t—  "~  —  \C.  

>c  to  cot-  x  x  i".  —  c  — 

x)oocx)oooooocooocJcn 

t—  CO  ~   C  C 1  X  lOH 

—  cicirc-r— ■irr;r;i- 
rjc.  rr;  c;  ^.-  c  rrr  -.  rr.  rr.  re. 
oooooooooc 

e.  ■  e  c  i  x x  —  —  x 

'e  xi  co  os  ce  — 

-  i  '—  !^  "1. 

— 

*  '"  —  i-  cc  ~  '~  —  i  -  :     ~. 
ci  do  —  —  ^ j  iC  co  r— ■  i>-  -X   > 

=}  x:  5!  35  CT-  CT.  CT.  CT-  ~  Cfc  33 

■  e  —  i  -  -r  c  c  re  c.  ■  e  c  i 
~.  c  O  —  ci  ci  re  ce  —  i  r 
rr.  rrcccccccc 

—   —  — *  *-i  i— 1  ^H  i— 1  ^-  ' 

x  —  —  i  -  re  e  i  -  re  re  I  - 
'_:  cc  (e  !_r  x  re.  re.  e 

-  i-  re 

_ 

i  -  i  -  y 

re 

i  -  --  c. » "  —  i  -  cc  —  *  c  —  i> 

0  —  —  CM  CC'  CC  —  -r  » 1  cc   — 
OC  —  c:—  ~  ~  ~  CC  CT  O 

d 

:  r  ~  ■ '.  7 1  x  -r  — '  i  - 
i-i-/:C. -Chhcmk 
OOOOOt— |i— Ii— iiH  — 

^1^-^-^H^^  — I- 

e  c  i  e-  i  e  —  x  .  -.  —  x  ■  e 
rr ■  -.  e  e  i  -  / 

—  x  .-  — 

— 
ci  ci  ci  ri  r  i  r  i  ci 

/  en 
—  >c  -e 

Cl  Cl  Cl 

ci 

CC 
s 

Cl  X 

s  c 

1 —  — 

-F  C 

c  — 

c 

1- 
c 

cc 

- 
cc 

^ 

—  i  ~  re  c  c  ci  rr.  •-  — i  x 

'ri.r  wNi-  y 

— .  —  T.)  -) 

'       *—*  ' —  ^H  r-i  ^^  ^T^  —  rH 

■tONMwcOM  r.  e  re 

-  -  c  i  r  i  re  re  — 

ci  ci  ci  -i  r  i  ci  ci  r  i  ci  ci 

:  re.  e  re  re. 

i  -  x  r..  _■.  re 

ci  ci  ci  ?i  re  rr  re 

- 
_ 


■re  e  ci  x  —  e 


ci  x  —    e> co i 


^LTLZer-Ei^^fir-Ii    ^^^^^     re  re  e  ci  x  e  c.  x  ,r  c, 


-  I  Cl  Cl  Cl  Cl  Cl  Cl  ci  c"i  cl 


re 
- 


j  x  -r  _  e  ci  x  —  e  -c  ci 
T,  -^  ^-  —  —  '  ■• '  -  x  ~  e:  e 
ci  ci  ci  ci  ci  ci  ci  ci  ci  ci  cc 


3 

- 


■  --  _ ci  rr  re  —  ir. 

ci  c i  rr  rr 


x  e  ci  x 

■  r  e  i  -  i  -  j   ~  ~ 
re  re 


-  ^ S t: E2 S  —  T1  x  '"    —  '  —  occoocowc    c  —  re i - 

S  ZZi  £J  £,  ES  ^  :              -     i  -  ;  -  x  _.  r..  e  —  -  =  - 

rccccococococcrcrecc     rrrrrr —  —     re :_e-  _  _  re  ce 


^-li^'i--'^-^^     -"icqeOrH-CO^O 


?r--r  —  —  —  —  —'-:,-• 
ci  ci  Cl  71  ci  ci  ci  ci  ~"i 


3    c^^^^^iciclclc^ 


—  ci  r-  —  ■rel- 


et-     l-l-l-l-l-t-t-l-t-X 
Cl  Cl  Cl  71  Cl  Cl  Cl  Cl  Cl  Cl 


RESEARCH  METHODS  IN  STUDY  OF  FOREST  ENVIRONMENT.       183 


2 
is  i- 

S  C3 

s- 

PH 


^OOOOCOOOO 

Oooooooooo 


.HNW^iflfflNWO 


o 

si 


'O 

'  o 


- 


cm 


q 
55 


O 


■  t  -      /.  /.  3  O  h  N  N  M  -t  LI 


'  o 


i.-  C  «  OS  "H  n  r:  -r  t  '"      CCNXOIOOHINP) 
QOOOi-(i-(i-l>-ti-H^H      —  .—  —  —  —OICMCMCMCM 

oooooooooo oooooooooo 


■  o  OOOOOOOOOO  oooooooooo 


-1- 

o 


-— . 

-1 

- 

:-. 

J 

- 

•  » 

H 

A 

^ 

0 

-_7^ 

/ 

C_ 

71 

^ 

PQ 

- 

s  > 

■- 

^— - 

pq 

~~ 

?- 

-' 

H 

> 

TO 


as 

c_ 


•  — 


- 
=s 

CO 


£ 


- 
- 
< 


q 


q 


ei 


~ 


'  CO 
■  71 


•r  i-ctoNXXOiOiO    rtHNraTi"i"tONao 
cm  oi  cm  cm  cm  oi  01  01  cm  co    cocococococococococo 


■re     rorerorerererocococo 

'  o   oooooooooo 


SOiO-C'lcorO'rL.-S 
re  re  —  t*  -*  -~  T  -r  -r  t 
OOOOOOOOOO 


Or HhWiOMONCC     O  I HOWiQMOOOin 

io  ic  O  r—  t~  00  o  O  O  —  CM  oi  re  —  —  >-  EC  f-  r  ~  X  ~.  C  C  -  c-i  ^  re  -L-;  --  t-i»-/oOHrfCi:-- 

7i~i~i~i~i"i~ir7~- — o  coco  co  o —  ro co co co w  rO'^^Tf'rf-rf'r-r^r'-T    — h  —  ioioio>oioi^ 

C  O  O  O  O  o  O  O  O  O  —  —  —  —  O  O  o  o  O  O  o  o  o  o  o  —  —  o  o  O  OOOOOOOOOO 


»ioNo>et«joooiOH 
eire— ■  —  leoi-t-  x  ~ 
rererererererererere 

oooooooooo 


/■::ir.i  — MX<OK 
— .  o ei  ro f  >o  tc 

re  —  — *  —  "T-  —  ~ *■  — h  ~"*  ~~ " 
---------- 


hZx.r.;r.r.oit>!-r    i  e  ■  e  co  i  -  x  oa  ca  o  —  01 
—  —  —  —  •"  '*  i"  '".  c  '"     » e  •  e  •  e  t  e  •  e  ■  e  •  e  co  co  co 

----————--     OOOOOOOOOO 


o  —  7i  ?i  re 

— 

o  o  o  o  o 


y    -  -    y~  — 

—  — c  —  —    "'r  ■+  —  ^  -t- 1*  it  i"  '*  »7    ie  ie  ,:r  ie  •  7  ■_:  <  7  o  cr  ^ 


or- —  ■-:-.  —  —  r\ 

r  -  i  -  x  o  ca  —  —  eir 


I  CM  ' 


-;i-/3rjc-N 


WMfulOI^NXOiO 
EC   EC   CO  EC  CO  tO  O  tO  tO  I> 

OOOOOOOOOO 


o 

iq  cm  ca  co  co  c  r-  '0  .  i  y     ■:  nr. '7--  /  c-7 

y.  ~  oa  O  —  ei  71  r7 i»  op  to  t-  00 oa  OS  © »-t  CM     re  —  —  >'.  cc  t-  i~  x —  O 71  re  —  lo  i-O 

—  — h  -—  m  ie  i7  17  ie  ^_7  in     '7  >e  ic  <e  ic  in  in  co  ^  co     co  o  cc  eg  o  cc  cc  c~  c_  i>-    r-»  ir*  !>■  tr^l>"t^t> 


I-  I-  r~ 

o  o  o 


:7    77   I /    .7   77  O   O 

o  i  -  i  -  y.  — :  —  o  —  ei  e i 

'7   '7   i7  »7  17   "7  'C   '  ~ 
------- 


coor~'rc^ioo-r'Mo 

re  —  —  >e  vr  o  i-  y  —  o    —  ri  ei  re  -r  ie  >e  o  i-  y     7.7.chmk  re  —  '7  ic 

©jpcojotoSiotpeptc    i  -  i~  t-  i-  i  -  i-  i  -  i~  t-  i~    r  -  i~  oe  x  x  x  x  x  x  x 


o  o    o  o  o  o 


oooooooooo 


—  y  ■  7  7 1  — -  o  re  —  y  •  7     7 1  r .  o  re  —  y  ■  7  re  —  V» 

-r  -r  ' e  o  o  i -  y  —  ~  o e i  r7 r«  »o  to  t- 1-    r.  o  r; 


-r  -r  ■  e  o  o  i  ^  y  — .  o.  o 
to  cc  ec  -  tc  •_:  tc  -_7  tc  t- 

ooooocoooo 


r-  r-i 

o  o 


—  ■  7  --o  t -  i^    — ■  c  o  —  ei  co  re  —  '7  '7     tc  o  i-  x  o  o  o  —  ei  re 
i~  i  —  r—  i—  i  —  r—  t  —  r—    r^cecccoooooocoooox      iooooooox)oao>C6«oa 

ocoooooo    oooo-^oooor         :    dooooooo 


oi —  y  ■  7  7 1  —  '-:  r7  —  /  i:rici--  noi-  —  —  —  —  —  7 1  r.  i  -  ■  7  e  i  e/.c-nc/.c-Tri 

oi  oi  re ioOcONOC  rr.  —  —  —  7  i  7  i  r7  —  ■  7  1 7  o  t  -  i  -  y  o  O  o  —  e  i  re <-.  \z  i~  s   ST-  O  .— i 

t^  i^  r-  t^  i~  i-  i-  r~  r-  t-  t^  r^  y   /   s   /   s   s  /  y  y    '  /  ss  —  —  —  —  —  —  ~.  o  o  o  O  O  ca  O  O 

oooooooooo  oorzrrrcnooo  oooooooooo  oooooooO'-irH 


JCOMOt- y   '7  71  77.  O  re  77    y  '7  71  O    y  '7  71  O    y  '7   r7  —   y   '7   r7  —      r.  I  -  '7  77  —  —-  I-  '7  CO  •— 

rrs  o  —  7!  oi  re  —  -    '7  o  ;i-  /  *  r.c-rin::  —  '7  '7  o  i  -  y  y  rr.  o  —    — ri:"-i7'7Ci-x  " 

\~srss////f  /  s  s  s  s  t-  —  —■  —-  ■?■  —  ~  saoictaaaaoo    oooooooooo 

OOOOOOOOOO  OOOOOOOOOO  OOOOOOOO 1 


r-  t  —  y  '  e  o  i  o.  y  re  o 

r  -  y  o.  o  o 


o  o 


/  •-.  71  o  i —  —■  r  -  — 

"NOCOCSQh 


—  r.  t 7  01  O  I  -  '7  77  o 

oi  oi  re  —  >e  o  o  i^  y.  o 


y   O  —  7 :  o  y  O  -r  CM  o> 

77-  O  —  0  i     - 


■  7'  re  re  —  '0  o  o 


y  y  or.  o  —.  y  o.  ~    joiaagocooOc    oooooooooo    o  — r-i i-h .h 


■  7  .    7T.  i~  —  —  x  >o  oi  ~  o  re  o  t-  17  oi  o  t>-  '-e  oi  er.  i^  i.e  oi  o  y  '7  re  —  x  o  -r  oi  o  x  o  -r  oi  o  X 

I777I-/7.  7.7  —  —  OI  77  —  —  >  7  O   O  I  -  X   77.  77.  O  —  01  re  re  -r  ■  0  O  O  I  -  X  O  O  O  —  01  re  —  — 

77. '.  OOOCnOOO  OOOOOOOOOO  Oi 1 rH  i-H  H  i-l  ?H  — <  —  r-7171  71  OlOlCMCM 

OOOOCOO' —  ' i-l  iH —  i 1 —  -<  -h  .-(  i—  — —  -, 


x  in  oi  o  o  re  o  r- 

77  —  —  '7  o  o  r-  x  as  oa 

oooooooooo 


to  cm  c  o  -*•  —  x  o  -r  —    » to  ■*  i-i  oa  i? — fMCt^ 
O-^  —  oire  —  ~iooi^    t-  x  o  o  o  —  oi  re  -r  -r 

—  ,-.     >^  —  —  01  01  01  01  71  71  71 


■  7    r  7  — (  OI  r~  l-O  CO  r-  03  CO 

■  0  CO  h-  NWfflOHH  7>l 

cm  (M  oi  oi  oi  oi  re  re  re  ce 


O  r~  —  oi  —  o  re  o  r-  —     oi  ~.  o  77  —  y .  <e  re  —  x 

oa  oa  O  < 01  re  —  —  i-e    o  o  i~  x  o  o  o  -+  cm  cm 

—1  —  01  01  Ol  CM  CM  CM  CM  CM     0-5  CM  CM  CM  CM  o  i  re  re  re  re 


oa  co  co  i—i  oo  ic  cm  o  co  co    o  t — -  —  o  i^  •—  cm  oa  CO    CO  —  X  i-0  01  O  X  o  —  01 
cc  i>  x  ca  o  O oi  re    —  -r  'O  o  o  i~  x  —  ~  o    —  oi  oi  re  —  ic  '7  cc  l>  X 

OMCMCMCMCMCOcecOcOrO      CO  CO  CO  CO  CO  CO  CO  CO  CO  ^      —  —  —  —  ——  —  —   — ^  —  — 


cm  ca  co  —  —  x  ie  oi  o  co    re  o  t — -  01  —  co  —  oi  —■    7-:ioi-  w  oi  o  x  o     re  —  oc  i~  'O  re  —  —  i-  -o 

7i  re r  i.o  co  co  r--    x  o.  oc  o oi  re  —  —    'O  eo  r~  t>- oo  eta  C oi    re  —  —  m  co  t-  x  x  ca  o 

oicMCMCMOi7i7i     7ioioioioioiror07ere    rorororerecocecocO'r 


ie  ce  —  x  o  ce  —  x  o  —    o i  o  x  co  —  7 1  o  x  -o  — 

r7  -r  io  i-0  co  i~  x  x  o.  o     —  oi  oi  re  -r  ie  co  co  r~  x 
re  re  re  re  re  re  ro  re  re  ~—    —  t-  ~~  ~  *" ■  *^*  "T*  "^t*  ^  i^ 


O  X  CO  —  OI  O  X  CO  -f  0-1 

o.  o  o  —  oi  re  re  —  io  co 

—  —  1 7  i-O  i-0  1 0  i  7  i0  1 0  i  .7 


r~  -r  —  O-  co  re  O  i 

-*  >o  CO  CO  I  -  X  OC  c  o  — 
cecererecececereT"- * ■ 


ca  cc  re  o  x  ■  o  oi  o  r-  ic 
—  CMre—  —  i  e  co  i  ~  i  ^  x 

_._. _  _  _  ^  —.  — . 


re  —  x  ie  ri  o  n  c  rr  - 
O-  o  o  —  oi  oi  ce  —  ie  co 

—  i7  '7  '7  i7  i7  i7  i7  "7  i7 


o.  n  17  r:  —  ca  r~  uo  co  o 
o  t-  x  oc  o  O  i-i  01  re  -r 

•  7  i  7  1 7  i  7  O  O  CO  CO  CO  CO 


SO  CO  O  X  i0  OI  O  t-  I-  o 

oi  ro  —  -t  >-o  co  co  r-  x  ca 


t—  Mil- lOOCOCOOOCtOCC      7  7.  O  -  01  OC  N  1.7  CM  O! 

oa  o  —  — i  oi  ce  -r  -i-  io  co     i -  i  -  x  oa  o  O  —  oi  co  ro 

—  i7  i~  i-O  i~  i7  i7  i7  i0  i-O      iO  i.O  i-O  i-O    O  CO  CO    O   -O  CO 


i  -  i.o  r7  — ■  ~  t—  i-O  ro  i— i  ca 
—  i-O  EC  I-  t>  X  0  0"  — 

co  co  co  co  co  co  o  t-~  r~  r~ 


—  —  x  co  cm  ca  o  —  —  y 

O  i— i  —  CM  CO  CO  T  LO  CO  O 
lC  iO  lO  lO  IC  i-O  i-7  i-O  lO  i-O 


>-0  CM  ca  co  -r  —  x  co  -r  —  x  co  —  —  ca  t — r-  CM  O  x  co  -r  oi  O  x  co  -r  oi  o  x 
r-  x  x  o  o  i—i  •—  oi  ro  -r  t  i-7  o  n  i-  z  o  o  -  -  71:7-17  17  o  i~  x  oa  oa 
10  i-O  »-0  io  co  to  to  to  to  tc     -7  cr  cr  ■;  o  o  o  n  n  h-    r-  r—  r*  t^  c^- 1^  t^  r^»  r-  r- 


- 
— 


—  01  re  -r  ie  co  t~  x  o  o    —  01  ro  -r  1.0  co  t~  x  oa  o    — ■  01  ro  —  •'.  s  i~  x  ca  o    —  cm  ro  —  10  co  1-  x  oc  o 


X  S.    /    /-   f-   /    s    /    /    ~. 

O-l  OI  OI  CM  71  CM  CM  CM  CM  CM 


CaOOOCaCaOaOOaO     O  OOOOOOOO— '     —1— 1  —  —  -h^-— h  —  —  01 
CMCMOiCMCMCMCMCMOire    cererererorecorerero    CO  CO  CO  CO  CO  CO  CO  CO  CO  CO 


184         BULLETIN   1059,   U.   S.   DEPARTMENT   OF   AGRICULTURE. 


- 


- 
z 

'- 

I 

o 


~ 

g 


c 

eo 
■'. 

•- 

ft, 

•■r. 

C 

s 

■  — 


X 


z 


ci 

CO 


- 
_ 
1     - 


— 
— 
C 

v. 

•C: 


CO 
cc 


O 

ft. 


Ci 

- 


q 

CM 


O 

d 


— 

• 

\ 

.    .    .    .  -f 

!    !    !     '■  c 

•  d 

. -.  ' -  ■-  r  -  x  rr.  ~  c  —  c i 

;' 

; 

.     .     .     • Ol 
.      .      .      -<M 
•      •      •      •  C 

;  ;  !  id 

cere  —  '  c    ~  i  -  i  -  X  ~  cc. 
C  i  C 1  Cl  CJ  CI  CI  CI  CI  CI  CI 

22.0 

)C  /  —  — ci  cc  cc  -t 

cccccccocc 

ic  ^  r^  r^  x  o  o  c:  — ^  c< 
_;  r.  —  —  ,-i  -j  ci  ci  ci  ci 
oooooooooc 

co  cc  —  >c  cc  i  -  x  r.  c  c 

Ol  C-l  CI  CI  CI  CI  CI  C  1  cc  cc 

cocccccccc 

-i: cci-  x 

c 

ci 

-   r  -  X    —   T-   — C 

HH SNNCj 

pn  r-H  i.c""C  ^  t^  cc  X  O  C 
CI  CI  CI  CI  CJ  CI  C<l  CN  CI  cc 

ccccc:ccc: 

-—  —  ci  cc  —  >c  -~  r  -  x  x 

coccccccccccccccccco 
cccccccccc 

cc  —  c  —  ci  cc  cc  —  ■-  iC 

cccc  —  —  —  —  — -  — ■ 

cccccccccc 

c 
d 

CI 

-*  •*  it  ■—  r^  t^  '/  0  0  c 
MC«<N?5<N<N<Nc3eoc2 

ooccccccoc 

— j  <m  co  co  ■*  lc  :c  a:  r^  x 
-^cccccccccccccccccc 
oooooooooc 

O  O  C  —  C 1  CC -iC'C 

cccc —  — ---  — 

■^  ^z~  ~  ~  —  ~  —  ~  — 

ZZ  ~  ~  ~  iT  '3  '5  'J:  'i  'J; 

0 

1-1 

-.-rccorccccccocococo       ^Z~ZZZ~Z~-tZZZtZZ~l 
OOOOOOOOOO      c  ~  ccc  _ 

1-1 

■-  ■-  i-  x  cr-  c  —  ci  cc  cc 

-—-—  —  -—  — -iC'C'CiC'C 

—————————— 

i ;  -.-  i  -  x   /  r.  ce- 
lt 'C  'C  '"  'C  iC  'C  'C  tC  '" 

q 

s 

— 

0  or.  c  — 

2  S  r~  — 

d 

- 
— 

- 

ci  cc  cc  —  iC 

—  -t-  —  —  — 
ccc:: 

S5h7'/JOHHNCC 

^+.  —  -*  --  -c  »-C  i-C  lC  lC  ic         »C  iC 

~  ~  ~  ±  —  —  —  —  —  c        OC 

i-C 

c 

■- 
- 

i  C  •  C  '  C  '  C  "CC  '—    '~ 

ci  -i  re  —  ■-  cr  -  i-  «  r 
a:  cc  cr  a:  c  -r  c;  c  ;    ■ 

q 

r-  r-  x  ~ 

—  —  —  — ; 

d 

o  h  -j  ci  «      tic--  ;i-zcjc:c;-       CI  c 
,-=  ,.  ,_  ,-  ,-       ,-  ,-  ,-  ,_-  ,-  i~  ifl  lococo       S  jr 

OOOOO        0000C:C:0C:00        OC 

—  .  -  -r  i  -  x  ~  r. 
cc  -C  --C    -          - 

c  c  o  c  c  c  c 

C  C  —  CI  -c lOcOl— 

t-i~r-i-i-i-i-i-i-i- 

—  —  —  —   --  —  —  -  — 

q 

d 

i-H 

,~  ,~  mi^cooc c  — '       01  ro  —  —  'C  cc  1-  rococo       cc  C  —  ci  cc  —  —  •"  —  t- 

0000000000      oooccrocroso      c c a 

l_2  l^  C± -7    £    '/    >     /    X    X 

'C 

— 

MM^"5<0!ON  r-  X  CC 
;__  ffi  ._  35  0  O  CD  CD  CO  CC 
OOOOOOOOOC 

0 

—  —  ci  ci  rc  -c  'C  ic  r  i-       y   x  ~  c  —  cicicc  —  'C 
rcz.  r  —  t -—  t^  t^ —  t-—  t—  r^  i^  t  —       i  -  i  -  i  -   '  s   /    s   /    r  x 

C.^.  —  ~~  —  —  ~~—        ~  —  ~  —  —  c  ~  ~   -    - 

c  vr  i  -  x  —  c  —  —  c  i  cc 
x    x  ^r    y    y   —  -   -   — .  r- 

11.0 ! 

_j_4<Mc«Tfl-*iou5c©f-      30ej»oorHCMcoeoTi<««       r  cr  1  -  x  or.  c  ~  —  r  1  : c 
7Z  7Z  ioi  r-  t-  r-  i~  r~  r~  r-      i~  1-  x  x  x  x  x  x  x    x        x  x  x  x  x  -  35  -  or.  or. 

oooooooooc      ——————— -~—      ———-——---- 

d 

-  -  '  c  c  i_-  /  /  r.  c  - 

13.0 

aicsOHNNccco'Cis      eg  i  -  x x  or.  c  —  —  -j  r_c       -~2^]-:LCZ~  — 
Kn//  /  /  x  oc  x  x       55 00 00 00 oc  ~  cs  aa  or.  go       ft  or.  or.  . 
0000000000      ;c:c::cc;:       —  -  —  — —  — 

d    "    ' 

>  "_    J   r  S  -   £  -  ^  2 

q 

cm 

— 

tOCONOOOidO^HM        »#iOcOCD!--OOOJgiOO        —  —  c-J  re  — ;  'JT  'T   r  i_j   / 

s  s  x>aoaoooa>OiO>o>      0  0  0  0  0  2  s  £ - 

;ic;:::oco      cccccccc 

0 

SOC  —  ciccrc  — 

11.0 

—  —  .---  i~  r— 000000      —  ci  cc  rj  —  'C  a:  a:  i^9£       £  ~  —  —  ?l  •"•  ""  "*  "■  — 

rjboooooooo  —      —  —  ^  —  c  —  —  —  — -  —       —  —  —  ■ ■ —      — 

000C—  00c;  —  - — — 1 — — ~ 

0 

i-i-  x  —  C ci  re  — 

TiTinri~iri 

10.0 

NCMCO,*'O'OcDt~-t~0C        ~  ~ ~i  rc  —  —  > "  \z        Nt-ZOC ?  1  cc  — 

i;  —  OOCOOOOC:         C NNNNNN 

. —  —  . — If —  ^  .— '.  ^-i  t-H  T— i  .— ^          f-^T-H^H^-^T— (r— I^HHH           -m^-.^  —   —  —    —   —   —   — 

O 

.  -  ■  -  s  i  -  y  T-  —  —  —  c  i 

CI  CI  CI  CI  Cl  CI  CI  c- 

q 
d 

rZ  S  1-1  —  — '  —  — 1  1-1  —  ^h        HHHHMNNNNCil         NNNNIMNNWMK 
O 

—  .  -  z  i  -  i  -  /  r  r 
m  -^  -                   -    -  -  -  — 

c 
• 

CO 

»00,050HHNCOCO'*        iQfflt»N»fflOOHN        CC  cc  —  >~  -~  I-  r  - 

—  CI  CAl  O)  Ol  CN  CI  CI        CI  CI  CI  C>1  CI  CM  CC  CC  CC  CC         cc  cc  ?c  CC  CC  CC  :c  cc  -c  — 

~i--  —  --.-^['y 

— — 

0 
1- 

-  c;  i -  /  3)  is  o  h  w  ci      cc  —  ' ~.  ' c  -_:  t -  ~f.  s  o  ~ ci:c-'C  'C  •-  i-  / 

N  C-l  d  N  CI  CI  CO  CC  CC  cc       cccccccccccccccccc—       — 

d 

-  r  -—-'■■■-—  •-.  — 

'C  '  ~.   >  ~   '  " 

q 
d 

—  —  i.c  03  t^  i^  y.  35 oo      —  ci  cc  cc  —  i.c  -©  j:  i »  /■       —  —  ~  —  ci  cc  cc  —  <c  -.r 

COCOCOCOCOCOCOCOCO'*'         ■*  —  — —  — -iC'C'C'C'C'C'C'C 

o 

'.:.:--  rj  cc  — 

q 
10 

HHcicc-c-ticct^x      oo  —  — <oico^r—  •'.  ■-       t~ t~ oo as c ci::  — 

-*  ^f  —  -r  -r  -f  -r  -r  •*  tji       ^i  io ii>  iq iO  iR  ifl  >c  >c  'C       ujiou;  ic  —  -^  ■_;  •_:  te  '-C 

rHr— IHHHf- If— <i— IHH          i — 1^-lT-Hr^?— Ir— 1— It— 1^^— ^           —  —  —  — '  • — ' 

o 

c  i  -  y  r.  r.  c  —  c  i 

cc  r  -r  ■-  ■ s  —  -  i  -  i  -  i  - 

OiSCHNNMiiiSO       I^XOKSOHNCICC-        lOiOtfiNOOOftOHN        :c  :  c  —  ■  c    c;  i  -     .         -    3 
^rf  -^  ic  to  10  10  ucj  10  10  ire;       iC  »0  lCj  ic  o  to  C  T  c  c        cc  cc  -^  -^  ■—  •—  -—  i  -  r  -  i  -       r  —  t  —  i  —  f  —  r  —  r  —  i  —  i  — 


t^  t--  x  o  o  o  i— i  (N  c<i  co      -f  '"  cc  ■—  t~  x  cr.  — ■■  —  —       c i  c i  cc  —  >  ~.  cc  cc  i  -  y  ~.       c  c  —  c  i  :c •  '■  —  i  - 

SioiOcOcOeocoOco       cocococococococor^b-       t~  i~  r-  t^  t^  i~  i~  i-  i-  '-        x   y :  x  x   x   x  x   x  x  x 


"CiCCN'/'/COOH         CMCC  — —  it  CC  I-  I-    X    3J 

c  cc  c  c  c:  cc  cc  n  i^  n       i>- 1^- 1^.  t —  t —  i>- 1>- 1^- 1^  t— . 


c  c  —  ci  cc  —  —  i :  ■—  i  -       y  x  r .  c  —  c  i 
x  x  f  s   /   /    /   /    r   r        s   s  j  ■?■  —  T- 


f2f5  ~    •  c  cc  i-  x  x  —.      o—;  cm  ci  cc  — ■  ic  ic  cr  i-       r  /  —  ~  —  ci  ci  cc  —  '-       tc  cc  i-  y  cr  r  -  —  ci  cc 

1  1-H  >-l  i-H  i-H  i-l  i-H  rH  ^j^h^i^^i^^,^^^ ._ CICICICICI 


eicccicjco-*l'3!ioocoi~~      xacoHOMMitic      cci-/  ccc  —  -ire       —  —  ■  c  cc  i  -  x   y  ~  c  — 

co  oo  ex  oo  c*  oo  cc  oo  oc  oo      ocxcooicidcicc       ---.—.-.-  =  —  ic  =       c  c  c  c  c  c  c  c 

i-h  i-t  t-i  i— i  th  i-l  i-H  i— i  i-l  »h         i-H  i-H  i— I  ,-h  i— I  -^  ^h  ,-(  -m  _ i         _<  ^  _ ^CICICICICI         Cl  Cl  Cl  Cl  Ci  "1  C  I  Cl  Cl  Cl 


33 


HNnt 
ci  ci  ci  c. 

CO  COCO  CO 


i  c  cr  t^-  -r  o>o 

ci  ci  ci  cm  ci  CO       CO  I 

COCOCO  CO  CO  CO         CO  i 


CO  c 
cc  c 


Ifl  CC  I-   X   c. 


CO  CO  I 

CO  CO  I 


— 


—  Cl 


CC 


J  ,-  -    -       _  - 

^— -  —  —  -*  —  —  .C  iC'C'C'C'CiCiC'" 


RESEARCH  METHODS  IN  STUDY  OF  FOREST  ENVIRONMENT.       185 


^3 

^   o 
.  „  h 

to    o 

■i 


- 


24.0 

ocoooeooooo 

ryn  ^vh  hv*  CO  hh  ^/^  £v^  j*h  71H  CO 

CHMM-t^LiaCN 

xcr:rz— i  n  m  ■*  lo  s  t» 
-r  ^1  i.e  i.e.  ».o  >e  »e  >e  »e  »e 

3C:OOOOC;CiOO 

c 

o 

co 

£\J  m  jo  ~^  n^  *n~  r^i  ^n  c*^  CO  'O 

ooooooooooo 

r^HH.H.Hti.HMHf-HH.H-.HHT* 

cooooooooo 

if  if  LO  L?  LO  10  iO  to  uO  iO 

0000000000 

©NKaOHNW^iO 

iCC'^lOtOlOOO'ilD 

0 000000000 

o 

* 

o 

<N 
IN 

,^H^_—  —  ^^.^VH..— 1— H-H 

ooooooooooo 

ONXOO-tHNM'* 

oooooooooo 

IC  L*  0  lo  l*;  LO  c  C  O  w 

oocopooooc 

o 

21.0 

■* *  **  t  *'.';  c  li  >e  ic 
ooooooooooo 

<  e  ic  » e  » e  '"  '*  c  ;c  O  to 

oooooooooo 

CO  ■*  »0  tO  t-  *«•  OO  OS  O  iH 

0000000000 

NCO'l'LOS^X^CH 

r-  r-  t-  t—  1^  t^  t^  t^  x  x 

ooocoooooo 

o 

20.0 

co  -f  io  i.~  to  r~  x  x  o  c  -* 
ic  >e  '*  »e  »e  »e  >e  »o »e  o  o 

ooooooooooo 

(M  K  ^r  u-  ■-  N  t^  X  »  O 
C  C  C  C  C  C  C  C  w  N 

oooooooooo 

oooocooooo 

C^N«-l"iOONXO> 
X   X  X   X   X   X   X  X  X  X 

=;  —  00000000 

o 

o 

05 

h  n  «  k  1 1-:  c  c  t^  cc  ai 

to  to  to  o  o  ~  -.r  -^r  •_;  •_;  o 
ooooooooooo 

O-NMTtiCL-SNX 

OOOOOOOOOO 

r^  x  x  x  x  x  x  x  x  0 

s =00000  000 

XXtt©3fflOS©»© 

0000000000 

o 

o 
X 

I— 1 

ooooooooooo 

woot-iefleoeoTfusco 
t»f»ooocoooooooox  x 
0000000000 

NxaoHHNM-tii: 
x  x  x  rr.  r:  r;  =  o  ~  o 
0000000000 

SNXfflOHNN^lC 
O OOOOOOOO 0 

OOOO^H— H^Hi — 1—^. — 1 

o 

o 
l~ 

i>.xooo—iCMc<icoHfio 
i~  r-  t-  t-  s.  y   •   s  s  x  x 
OOOOOOOCOOO 

-—  t~  x  oj  o  — <  —  ci  re  -f 

X    X    X    /53iffiO  O  crn 

oooooooooo 

■  :CNMaO!OH(MW 
OSOSOSOSOSOSOOOO 

*lf!CN0C'OO^N(0 
OOOOOOt— ii— li— I?— 1 

— Hi— «— hi— (^-^H— H^H^Hl— 1 

o 

c 
d 

t— 1 

00 
i.~  <o  t^  t-  X  C.  ~  OHNM 

oo <x  x  x  OKaoaao 

ooooooooooo 

-r  >  e  o  i  -  a  r^cicr^-'Ci 

;.  c.  ~  ~  ~  s  0  c  c  c: 
occ~~o~  —  1—1— 1 

::-iC£M^XfflOH 

cooooooo 

T-l    1— 1    1— 1    1— 1    1— ll— 11— ll— C^"l— 1 

t—  h-  —  —  —  — 1  —  rtC^CM 
1— 11— l^-i— ll— 1— Hi— 11— l~Hi— 1 

o 

1" 

1—1 

CO  -r  io  "0  eo  h-  0C  00  O  Q  iH 

oi  o^'  ci  C-  c.  c  c  r.  r.  c  c 

IN  CC  -M-t  C  N  1^  /.  0  c 

c:c;c:crc;c:c:c:c:  — 

r-i  ci  co  **"  >c  >r:  co  1^  x  x 

CCHNMifOCI-  X 
— <ClCMCS!NCMC4CJC)Cl 

o 

o 

— 

h  fi  ?:  ^  -r  i-  o  s  t^  x  © 

ooooooooooo 

Oi-ic<icoi*,ioiocor~oc 

- :  ~  — '  ci  re  re  —  i- ■  o  O 

—  Cl  CI  CI  CI  C4  CI  CI  CI  CI 

1-  X,  ©OHNCOt)llO(D 

ci  ci  ci  re  re  re  re  re  co  co 

o 

o 

co 
1— 1 

O  O  r-t  i-TOa  CO  Ti*  *#  IO  to  to 

O    hh   ,—   i-H^h.— 'i-Hhh— H—- 'hH 

1-  /  ci  ~  —  ci  Cl  ro  —  ■ " 

--rt^iri  Cl  CI  CICI  Cl 

oicacic^cocccocccoco 

>'.  =  e  -  x  O  O  -h  0  ce  -r 
cerececoco"i"*t,'t''t,Hf 

o 

o 

<N 

1— 1 

(CNOOXOCi-IN?:-- 
»—  .—  —  —  —  Cl  CI  CI  Cl  C)  <M 

'--.ri-  X  rr.  cchinco 
<m  ci  ci  c:  ci  rr  cc  cc  cc  cc 

-f  m  tC  h-  X  X  OS  OH  tN 
rerererererere-r 

co rfio to t--  x  acrfiN 
—  -r  — — 1 — ~  -v  -f  >~  ie  'e 

o 

11.0 

—  •-.    S    S  1  ~    /    — 1  ~   —  CI  IN 
<N  CI  CM  <M  CM  CI  ci  co  ce  re  :- 

cc  —  io  .r  1  -  f  s  —  —  — 

c  1  re  -«■  i.e  0  0  r-  x  0  0 

^^        "     ^T*    ^^*   ^^    ^^   ^T   ^T*    "f    ^-  • 

iHf)nt«oNxcoo 
1  e  >c  >c  lo  uo  ue  io  10  10  o 

o 

o 
d 

^H 

N  K  -  -r  •"  C  1-  "/  tt  C  C 
re  re  c~  re  ce  re  re  re  re  -*■  -r 

—  Ci  re  —  •-  it  \z  1  -  s   — 

—  ~~  "■  "*■  "*"       ~"* ■  ■**■  - *■ 

5  —  7  1  EC lOtOt^X 

i  ~  i~  ic  Lt  u:  c  1"  1*  ■  ~  1  e 

SCniMK^LOtONX 

1  e  ■—  hOOO^ccidcd 

O 

-  —  —  —  —  - — 

~  c  —  c  1  y  —  —  '  -  -,:  1  - 
~  ■  "  ' "  •  ~  •  ~  '1  1 '  IC  • "  '  ~ 

x  — :  cr  —  ci  c :  re  —  ■  -  ■- 
U0  UD  0  to  to  to  to  tO  tC  'S 

cc  to  cc  t~  t~  1-  0  h-  t~  t~- 

■  .  ■ 

d 

o 

00 

x  o  o  O  —  ci  re  —  ■  -  o  ec 

^(^IIOIQIOIOIOIOIC  »".  '" 

t^-orcsc^cac-irc-ri': 
IO  ic-  *c  to  to  to  to  to  to  CC 

—  ooot^-t^-r--t^i-i~ 

iOCNXOIOhKNCO^I 

r~  t^  r-  r^  t~  oo  oo  oo  oo  oo 

o 

o 

©Na»0>QrtN»T(l^ 

iC  iC  lO  IC  c  c  c  »  c  c  o 

IOCNMOCOHNM 
C  C  2  C  w  N  t^  1^  N  N 

■*iflCNXXOlOHN 

t-  t- 1-  r~  t-  t^  t^  x  x  s 

re  —  licnmcjohn 
X  X  X  X  X  X  X>  OS  OS  OS 

o 

o 

■ 

■*iO!OiONOOO>Ot-I(N<N 

lOOlSCCCCI-NNN 

cc  ■*  >-  tc  b-oc  z  —.  0  th 
t-  t- 1^  t^  t^  r-  t^  t~  -s.  oc 

C5  CC  if  LC  0  0  t^  x  0  0 
x;  X  X  X  X  X  X  X  X  0 

—  0  ce  -f  i-e  0  1-  x  0  0 

cr.  ~-  cr.  C-  e~-  c.  _    ~  ~  c: 

o 

q 

t-r-r-t^t-t^-r-r^-t^xoc 

-HClCC-^iOtCCCt-OCOl 

y  s  oc  oc  x  oc  x  v-  v.  x 

cr.  0  c.  0000000 

0  C  hh  ci  ce  -f  '"  0  r-  l~« 
OOOOOOOO 00 
i-<dlTMCICMCIC4iCICMCl 

o 

t-ooooooooooooocoooooc 

XOCHNMfO-tCtO 

x  x  0;  0: 0  a  cs  O".  01  © 

NXOCH-JlKTtL': 

OlCiOOOOOOOO 

or^xoOi-HCico^io 
cicicic5cjc<(Nc1ciiN 

o 

q    ' 

ooxxxo»aoic»c~.0) 

tSNXWCr-'HMMf 

OlOC.  o^cooooo 

C!SMKCSaOH«M 
OOOOOO^-ii—i— 11— 1 
CJCSidlNClClcMCKMIM 

-*i.OOSX©OhNCO 
1— ii— 11— 11— I1-H1— l(N(N(N(N 
C^INCNINININlNlCIINCa 

o 

o 

<N 

iCfflN  t-XOC--ClrOCO 

OIOICCISCCCCCO 

iHHl-tHlHrHNWNNlN 

•*iOt£NX©©CH(N 

occoc;c:Oi--i-i^ 

(MCJ<NC4(C4CN<M(MeN<N 

M^iOONNXOOH 

r-Hi— 11— li— 1— -— Hi— 1— 1  NCI 

cac*C1<Cl<NC)CNC-l<N<N 

NM'f  uCtCNXaOH 
Cl  IN  Cl  CM  Cl  Cl  C)  Cl  CO  CO 
ClClCliNINClCMINCaiN 

o 

q 

i-i 

C<5*iOiC!DNXC»OHH 
OOOOOOOOrHi-liH 
CN<N!NIN<N<N<NC}IN<NIN 

NCCfiCChMXlffiC 
^-— -^^-1— 11— 11 —  i-i-iCI 
M  N  IN  <N  CO  N  IM  CJ  IN  IN 

HNWfiOiOCNteO 
CNC<ICa<M(MCM<M<MCllC4 
<N(M(M<MCMIMC«CMCNCM 

CHINM^iOONXlS 
COCOCOCOCOCOCOCOCOCO 
IN<N!N<N<N(N<NC1CJ(N 

o 

o 
d 

•"■CMCOCO-iflCO^-XOO 

.— 1  1-H  1— 1  1— 1  i— IHHi- li- 1  1— 1  1— 1 

<N<N(N<N(N<N<N<NIN<N(N 

(M(M<M<C1CJ(N(C1I?<I(M(M 
(M<M(MIM(N<M<MCN(M(M 

aoHNn«-*wco^ 
in  re  co  «  «  co  co  ro  cc  ro 

INCNNININININNNN 

OOffiOHMM^iOfflN 
CO  CO  t<  ■* -t '*'*'*'*  ■* 
(NCN(N<N<N<NC4(NCliN 

O 

.a 

Oi-HCMco-^icor-ocoso 

i-KMC0'*>Ot0t-00C3O 

HdM^ioeNxao 

i-HiNeo-*i>oor-xoo 

tOOtOOCOCOOOOOt*- 

eocococo-cococococococo 

cococccococococccoco 

ooooooooooooxxooo 

cocococococococococo 

OOOOOSOSOOCJSO 
COCOCOCOCOCOCOCOCOTf 

186  BULLETIN    1059,    U.    S.    DEPARTMENT   OF    AGRICULTURE. 


a; 

\ 

: 


BC 

,. 

!C  o 

i- 
- 


s 

-* 

_ 

3 

_ 

■* 

n 

~- 

.- 

> 

k. 

i> 

— 

cc  -r-  >c  cc  i-  X  3JOH  ri 

M?l*l 

O 

DO  ~ "  '"  ©NOOCSOH  CM 
Cl  Cl  CM  Cl  CM  CM  CM  CO  CO  CO 

::  -1-  ■:  r^  X  ~  re  —  ei 

^r-'V-N/  cc  re  —  ci 

—  —  —  -»  —  —  —  '-.-.  •- 

cd 

^  j  ~~  —  1  ~  vc  r —  x  cc  re 

ci  CM  CM  CM  Cl  CM  Cl  CM  cm  co 

• —  ^l  -c  —  1*  i"  ^  1-  X   33 

O  — •  cm  cc  ■*  "O  cc  r~  cc  re 

—  —  - —  —  —  —  —  -—  -^  uC 

—  r  1  re  ~  ■  c  cc  1  -  x  r.  c 

'  ~  •  c  •  C  •  C  '  C  1"  •  C  •  —  •  c  cc 

q 

— 

x   — :  re  —  C !  r'  —  1  -  :'  r  - 
CMCMCOCOCTlCOCOCOCOCC 

x  r.  c-ri^-'"CN 

re  re  —  —  — —  -f  -r 

X>  OS  C  #-1  CM  CO  •*  ■"  t^  X 

— c  -—  1*  <c  >c  <c  <c  -t  »e  ic 

c.  cc  —  n  :  ?  —  '  *:  cc  r  ~  y 

0 

-r 

cc  t~  x  35 0 1-4 cm eo Tt« 1: 

cococore-r-r-r-?"-T"C" 

CC  t-OCO>OH  CM  CO  -P  ifl 

-*■-+•  t  -^  >-c  >~  >c  ■"  '~  m 

C  1^  X  ~  re  —  ci  re  1-  cc 

1—  it   1—  L.t   vC    vC   CC  CC  CC   CC 

1  -  X  ~   5  —  -  1  r-  —  ■-  cc 
;  cci>i-i.i.|-i>i- 



c 

7^; 

—  .  -  cC  1  -  s   35  O  1-1  CM  CO 

—  —   — :~    '~   >~.    ''. 

T»4lOcOI>-CX   ~  —  —  ci  00 

'  c  '  c  '  c  '  e  '  c  '  c   —   cc  cc  cc 

■tec  1-'/  3>  O  —  CO  *# 

cc  vc  cc  cc  vc  vc  t—  r—  r—  r-» 

■  *:  cc  t-  '/:  cc  ~  —  ri  re  — 
i~  r^  1-  t~  1-  j-  x   s  /   s 

~ 

12. 1) 

M  r:  -  '■  c  t-  X  cc  re  — 
IC  IC  'C  •—  'C  »C  »r  »C  CC  CC 

NM^OOCONK   ~  ~  — 

cc  cc  cc  cc  cc  cc  cc  cc  r~  t- 

CM  re  —  ■  t  cc  1  -  x  re  —  -m 
r^  r-  1-  r-  r ^  1  —  t  —  r-  X  X 

co  •"*  10  to  t~  oc  —  cc  —  ri 
/    /    /    f    f    r   s  —  —  ~ 

CC  cc  CC  *  CD  "X  CO  CO  CO  CO 

r~  t~-  t^  r-  t^  r^  t-  r^  r--  t- 

re  —  -1  :-  —  >-•-  1-  ~  — 

—  m  re  -*  1"  cc  1  -  k  r.  r 

_     3ft  ~   c.  c.  c  c.  c  _ 

0 

>:  —  cc  —  ?-i  re  —  ■-  co  t- 
cc  cc  0  r-  t>- 1- 1- 1—  t- 1- 


X  cc  ~  —  ei  re  —  ■  e  cc  i  - 
r~  t  -  x  x  y;  x  x  x  x  >: 


X  3SQ-HCMCOTl4iCr-OC 


r.  r  —  -_:  v:  —  ._e  cc  r_-  s_ 
—  cm  m  fi  ei  cn  ?~i  fi  c3  r~i 


cc  t~<X  cc  ~  —  ei  re  —  •-. 
t  -  t-~  t^  r^  s  s  s  x  x   x 


cc  1  —  x  C-  cc  —  e  1  :e  —  •  e 
X  X  X  X  3!  CC  CC  c.  CC  CC 


lONKaQHMKiQffi        l_;/  C  C-nrc-^c: 
CC.  C  d  CJ  ~,  ~|  — ,  — ,  5- j  5-,        CsJCMCMCMCMCMCMCMCMCN 


i*«5tSNOCO)OHINM 

x  x  x  y;  x  x  c  cc  cc  cc 


—  ■  ~.  cc  1  -  x  cc  cc  —  : 
cc  c.  cc  cc  cc  cc  cc  cc  cc  cc 

CM  1-1  CM  CM  CM  CM 


-■■  ci-  /  r.  c  -    •—         _ci-/  r.c-n::- 
rcrrrrcccccc cm  cm  cm  cm  cm 

.-IT-lCiriClCIC-IC-iriC'l        CMCMCNCMOICMCMCMeMCM 


NM-^OtDNOOCSCH 

C.  05  ~  CC  — :  cc  Cc  Cc  JC  CC 


■_  :j  —  ,e  cc  t_;  x  cc  cc  — 
C^ic^  ?  CM  CM  CM  CM  CM  CM  CM 


-irc-'CCI-    r    \- 

~\  ri  ri  ?i  ?i  cm  ri  ei 


—  ci       : •-.  ■  *  1  -  /  c.  cc  —  ci 

ci  ci       .  i  ci  m  ci  ci  ci  ci  re  :c  :c 

CICl         CICIClCICICICl.lClCl 


cc  —  ci  r-  —  re  co  t>  x  cr. 

~  n  ~  r.  ~ 1  -n  fi  n  ?i  -n 


cc  —  ci  cc  —  "C  cc  1  -  x  cc 
ci  ci  ci  c:  ci  ci  ci  ci  ci  ci 


re  —  cire  —  •ccri-cc  ~ 
c  icicicicjcicicicic- 

Cl  Ci  Cl  CI  CI  CI  CI  CI  CI  CI 


—  -I  --.—■-.  --    -  \  -  f  — 

-*  -^  ^  -^  ^"  -^  ^  -^  ■«"  -^ 
-.'l  CI  ci  CI  CI  C"l  ci  CI  CM  CM 


1—  x  cc  cc  —  ci  r-  —  ie  cc 

CCCCCC' —  1 — 

cici'.r.-i  .'incici  cm  ci 


r~  x  cc  re  —  ci  r-  —   -  z 
cicici  ei  ci  ci  cm 

Ci  CI  CM  C-l  Cl  Cl  CI  CI  CI  CI 


c  cc  —  -ire—  ci-       rcc  —  "    ■"—    ~cci- 

cicicir:  -        c-c-  — 

CM  CM  CM  CM  CM  CM  CM   CiClCI         CICICICICICICICICICI 


fc. 


v. 


- 


iC  CC  N  /  CC  re  —  CM  *•"  -*" 

Cl  Cl  C-l  CM  CM 

IC  I  C I  C  1  C I  C  I  C I  CM  CM 


>c  cc  t—  ce  c.  rc  —  ci  re  — 
ci  ei  ci  ci  r  1  rr  re  :'*^  ^ 
ci  ci  ci  ei  ci  Cl  CM  "I  CM  CM 


■  e  cc  1  -  x  cc  re  —  c  i  —  •  ~       cc  i  -  x  cc  re  —  cm  c.  —  •  c 

rerercrrrc —  —  —  —  .-.-,-.-.-,- 

CICICICICICICICICICI         CICICICICICICICICICI 


r-  —  •-  --  t^oOOO"  tN 

_ . 

^1  T-i  ?l  CS  M  <N  d  CC  D — " 

~. 

(N  CQ  W  (N  (N  C^l  r»  ^J  .1      i 

:?  —  •*  to  r-  s  zr.  o  —  ri 
"'  r:  re  re  ~~.  r~  r~ — 

Ti  '.i  '.vl  TI   /I  ?  1  CN  1  (  ?1  C I 


—  __  —  __-_-_.-,-,-         ii7  ut!  *C  i^  » '  C     C  *-T 

nriri  rinrin  ?i  ?j  ?i       :in:iri:i:i:i  nnri 


CM 

^h  t>3  cc  —  » ~  -jt  r  ^  y  35  ~ 
re  cc  re  re  cc  cc  cc  — 

ON  :M  0  i   .  1  '.  1   ,i  ["N  ri  ■>!  CN 

— 

3i  C  —  "1  ::-*c  CN"/ 
1 

.  l  C  1  C  J  Cl  *M  <N  rvj  ^  r« ,  ^ , 

—  ci  re  —  .-.  vc  r-  x  r.  re 
— -  — p ,- 

Cl  Cl   .  1  Cl  CM  Cl  Cl  Cl  Cl  Cl 


cc  re  —  ci  re  —  i.e  cc  i-  x 

-'Ciccirire'"!-^ 

Cl  Cl  Cl  Cl  Cl  Cl  Cl  Cl  Cl  Cl 


—  ci  re  -?- 1-  cc  i~  x  re  —  c  i  re  —  .  -  c  i-  x  —  re  — 
e  'C  •:  e  e  'C  <:  <r  c  c  co  "  r  -  C'~'~--f-i- 
cicicicicicicicicici       ci  ci  ci  ci  :     "   ?   -    -ici 


cc  re  —  c  1  r-  —  ■  e  cc  x  cc      re  —  nrr-ec:  i-  /  c 

.  •  cc  cc  cc  cc  cc  cc  cc  cc  cc        r  -  i  -  I  -  I  -  t  -  i  -  t  -  i  -  I  -  I  - 

CICICICICICICICICICI  CICICI   CICl   CiClCI-l-l 


!_:  i  £:  =  —  ~J  -  "f  '"  cc 

'.  1  :  i  c  1  c  i  c  1  v  1  c  1  c  1  ■. ■  "1  c  i 


1-  x  —  re  —  ci  re  —  ic  vc 

lC  1-  ic  vC  CC  VC  VC  CC  CC  CC 
CICICI  Cl  CM  Cl  Cl  CM  C-l  Cl 


1-  x  cc  re  —  ci  cc  —  cc  i  -       x  cc  re  —  c  >  c —  •  ~.  ■-  i  - 
cc  c  r-i-Ni-r-i-i-       1-  1  -  r    1    f    •    1    xxx 

Cl  Cl  CM  CM  Cl  Cl  Cl  Cl  CM   Cl         Cl  CM  CM  C 


CC 


—  ci  re  —  ie  cc 


c  1  re  — 


cc  1- 


cc  re 


c  1- 


**~    -*~J*     "~~     -™     •-*    — -     —     ^^     _ "" ". 

—  — ---recce—       —  -  —  —  —  —  —  —  —  ci  icicici-i 




RESEARCH  METHODS  IN  STUDY  OF  FOREST  ENVIRONMENT.      187 


— 
© 

3 


O 


i- 


— 
re 


-M 

re 


■-e 

— -    — 
— 


- 1 


r 
-i 


■  „ 

F- 

so 

- 

c 

© 

u 

— 

^ 

•  s 

eg 

v-*. 

- 

»-l 

SO 

H 

H 

£ 

^ 

— 

v. 

e 

►c 

■  „ 

-c 

SO 

r.-> 

« 

= 

'  — 

cq 

< 


© 

s 


—  "i  re       •*»-  it 

©  ©  ©       ©  © 


cc  r~  x 

©  ©  © 


c  © 


— i   M 

©  © 

©  o 


:  I1  ^  2  '^  2  t  2  ffl  ° 


— '  CM  re  "-t*  tt  t^  X         C3  © 


■-iCNeo^ioco^-oc 


r~  X  ©  © 


— i  cm  re  ifl  cc       i-  /  *  c  -  ri  c'r  -r  l»;  © 

—  —  —  —  —  —  -—  i-H  -M  ^l  CM  C-l  ""1  <N  CM 


ttOHNM        <*«3©NOOO»Oi-IW*        ij  i  N  X  3  C  h  M  M  tji 
—  —  — i  —  i — i  - — ^i  oi  ^  oi       cn  ~i  ci  r-i  fN  re  re  re  re  re 


r~   f  rr.  ©  — 


e  i  re  -*■  •  t  cc  i  -  y  — 


©  —      iNeo-*  u:  tc  r~  x  ©  —  -> 

.1'  ?_!      2J  7J  ?J  2J  ?_!  !1!  1.'  N  co  en 


ei  re  —  i-  ©  i-  x  —        SHNM'tin  ©  i~  y  -. 
—  —  — — < —      ~i  ei  -m  ci  - 1  -i  -i  ~i  -i  - 1 

©©©©©©©©   ©©©©©©©  ©  ©  © 


©  —  (N  00  "*  lO  CO  t^  OS 

r*~  re  re  re  r^  re  re  m  po  ■ 


—  ~i  re  -i-  it  i-  -.-  r-  /  3 


—  '  --  -r  r:  Z!  £?  I*  I."  —  '  -       y  ©  ©  —  ~i  re  —  .t  _t  1  -        <-  — 1  ©  —  ei  ei  re  ~r  r-  r~       MaonNo — H  m  co  r*- 

©  qSooSoSS  -'  D  S  S  2  3  ^  2  2  2       22^":r*~       -"  ^  •"  1 1  •'-  >'r  .•:  It  .~  It 

zi  rf 


—  1  -  v.  r:  r.  —  *  i  rr  —  •  - 

-*  t^  -<   3&0  •"■-  Cfl 

*  1  *  1  -  1  ~l  -■-  -- .->  -V 

--  -•*•  --  -.-—*.  —  — 

—  —  —  3  O  ©  G  *~c 

—  C:  ~  ~  ~  ^  z: 

■e       cc  t--  y  ©  ©  ©  —  ei  —  ie       cpt-»cooi©  —  ~i  :"  —    " 

ZT         JT^^^^lOlClCUili;  't  '~  C  it    "  ■*  ■"  -^5  ;c  :C 


—  '  t  \z  1  -  y  —  — 

7V;     -^    -v-     -^.     -^    ^«     .4. 

~  c  i  ir  i:  '—  — 


Z:  J  iZ      n:  LI  ^  L:  ^,  L  £  ?  ~  ~J       "-'■'.:i-/.-;:i  :t       —  .e  tc  t-  y  --  o  —1  ei  re 
~  ~  ~       22;SSr??IS  —  —  '-^  —  —  —  —  '£  '£  "ii       ix  iP.  —  '~  ~  —  '  ~  '  —  I*"  I*" 


—  —  —  TtTJTtT'trZ''^'" 


-  tirt-it  ri-  f  - 
•  t  it  1"  it  it  it  -t  't  it 


—       —  ti:t  —  1  e  ^  t -  y  c  —       ?ir:-«it  c  1-  /  ~.  ~  — 
©       "^  ©  j£  jc  jc  _::  cc  ©  h-  Is-       i_j  1^  t>-  i_;  t~-  i_;  r~  r^-  y  x 


©  —  ei  re  -r  it  ©  t~  y        ~  ~ 

it  't  »t  't  1"  't  it  '"!  't         IO  CC 


-  ~  —  e  1  re  —  •  t  •-  /  33       rr  —  ri  re  -f  it  ■£,  t-  y  ~ 
==  —  —  —  —  —'-:  '-:1—  (.r       7//^^///// 


1  -  y  ~  c:  —  ti  -."  -r  ■  t  j: 
'-2  '^  'J;  .;  £  '^  i  ©  =  ~ 


1  -  y  rr.  c;  —  ~  1  r  t  —  1 1  © 
j:  \z  cc  1-  1  -  1  -  1-  1-  1-  r~ 


'  -  '  ~  ~  —  - 1  re  —  ■-  1  -       y  -  rr;  —  - 1  re  —  ■  t  _t  1^ 
1  -  1  -  1  -  /  x  x  x  v   x  x       s_  s  ~.  —  rr.  rr.  33  as  rr.  ~ 

—  -~~~~~~~ —      rttcrccccc 


'2  -5  'r  ^  -■  -   —  '-''  -"  ~      '"  '—  i>  x  ~.  ~  —  ei  re  — •      it  ©  i^  y  rr.  ~  —  ~i  —  it       ci-  / --i- — r  ■- 

=  '£  =  £  £  fe  ft  fe  fe  fe       tttL-tliiii       r  //  //  g  03  CR  rr.  rr.       sofflooooooc 


re  -*  •  t  vc  1  -  y  rr.  rr.  —  7 1       re  —  1 1  • ■-  r  -  y  rr.  rr 

'^  1 1:  'r  !  •  'r  t  i  i  £       x>ccoo3cx>xx>cb 


t  -_r  1^  x  rr.  tr  —  ti  rt 


7  y  x  7  '7  x  y  y  x  «      -  -  -^  ?  si  -'-iS=      hmm^ioconoooh      nm^  ._t 


1 1  ©  1  -  y  rr  rr  — 


£ S co cb c»C6 0 c» ca en       rr  ©  rr  ©  ©  ©  ©  ©  ©  —       — © firirirititiriri  in  xi 


(-■  y  rr.  ©  —  ei  re  -r  it  ©       1^  x  rr.  ©  —  tM  :t  —    t  ©       1^  x  ©  ©  —  ei  r- — "  ©  1^       y  — :  ©  —  -1  ->-  —  •-  ©  1, 
©  C»  Cw  < —  —  —  —  —  —  ©       —  —  ©  ■ — ■ ti  ri  rt  ri  M  m  ri       ei  ei  re  re  r*  rr  :t  rt  re  re 


i^S2£r22S©'_,c<,S,^',•,      >t  ©  r^  x  rr.  ©  —  ei  re  -t-      iOconoocbOi-kn^io      ©t^xo©©— <  ~i  ^  -f 

—  —  —  —  — —  —  —       —i  —  —  —  —  ti  n  ri  tin       ei  ei  ti  ei  ei  re  re  re  re  re       re  re  re  re  -f  -p  -f  -r  -*■  —- 


—  7 1  re  -r  1 1  ©  1  -  x  33  © 
©©'©'©'  ©'  ©  ©'  ©"  ©'  — ' 


—  ei  re  —  it  to  1-  x  ©.  ©      —  ei  re  -*■  it  ©  t-  y  ©  ©      rn  ei  ri-ftsNMac 


—  —  —  -*•  —  —  -r—--^ 


ririririnri  tin  n:r 
-r-f  —  —  —  -*  —  —  -*  — 


-~  -^  -^  -~  --  -^  -^  -^  -^  — 


188         BULLETIN    1059,    U.    S.   DEPARTMENT    01    AGRICULT1   RE. 


s 

- 

CC 

- 

■- 

-5. 

/ 

^~* 

•3 

— 

'— 

■— 

H 

02 

o 

^~s 

— 

- 

-*< 

- 

'- 
~ 

rc 

£ 

■  -a 

- 

fc. 

-• 

pq 

: 

"- 

H 

x 

— 1 

- 

: 

£ 

-r. 

T 

&. 

-■ 

so 

- 

. 

- 

2 

- 
- 

— - 

a 

►o 

- 
71 

-  N»OH  m  --  —  '_£  2 

DO  05  C  —  71  77  —   -7  1  -   X 
i  —  71  71  71  71  71  71  71  CM 

77.  77.  —  77  —  —  '7    -7    /    7. 
^1  "^  *^  77   77   77   77    77    77    *  7 

77—  71  —  17  7  1-7.  77   — 

— "      " 

7 

-i 

71 

, Hi i — 

i"  C  t^  X  C:  C  —  C7  't  "7 
71  71  71  71  71  77  77  77  77  77 

CO  1-  X  77.  —  71  7'   —  77  1  - 

CO  :7   77  77   — "  — 

rH  —  . — |  , pH  1 

/  -no CMC lilt—  at  r 

—   —  '  7   ■  7  »  7   '  7   •  7   ■  " 

— 

C*  CO  iO  CO  t*  OC  3i  ©  *-«  CN 
N  ri  CN  r*  tN  (N  CN  co  M  cc 

, h    _,    _    ^H    ,-H    »— i    -—    — 

.•■•••••• 

77  -*■  '7  77  XT-  0C  C3  —  71  77 

77  77   77   77   77   77    77   "T1  ^T  ^* 

—  ' i —  t — 1  —  ' —  ' — '  ' —  ' — 

-f  i-7  77   X   77-  ~  —  7  1  —  i  7 
--*-i7^i7i7i* 

77  l~   X   77  —  7J  77  '7   CC  1— 

— 

co  co  co  co  co  cc    t  CC  CO  ~~" 

r  —  —  — '7  '7 

71  77  —   -7   l  -   f    77.   77  71  "7 

1 7  1 7  '7  1 7  '7  1 7  '7  CC  77   77 

—  .7  77    X   77.  77  —   "7  —  .7 

7    7    C   77  77  1  -  I  -  1  -  !  -  t~ 



L8.0      19.0 

/    — :  —  71  77  —  '7  77  t—  00 
-- > r  — 

—   —  i —  ~1  77  ~~  "7  h-  X   77- 
^7  i7  »7   i7  '7  »7  '7  '7  »7  »-7 

77.  —  7 1  —  •  7  -7  I  -  X   77  — 
77  CC  77  77    -7  77   -Z  77  1-  1- 

71  77  —  77  I  -   X    -    —    "i     " 

i~  r~  i-  t^  t^  r-  i-  x   X   f 

i-  ^  z  r.  c  -  m  ^  •?  >-o 
—  —  i-  >~  ic  us  ifl  m 

, — | ^H   i — I 

o 

r>aOO»Oi-ieMICO>OtDt- 

l7  .7  '7  'X  CC  CC  —  ~  —  — 

y    77.  77    71  77  -!■  •  ~.  CC  X  77; 

77  —  71  —  '7   77  1-77-77   — 
J0000O000000MO0O5OS 

o 

--  —  77  t -  X  35  O  —  CM  to 

i-  i-  i.~  >7  '7  >7  77   —  —  CC 

__  . —  , — 1  —  1 —  1 — 1 1 — I  < 1 — 

(75 

l0C0t-00C35O<-IC0,>5Hlfl 

tc  ;C  cc  —  —  t^  r-  r^  t^  b- 

77  r  ■-  X  77   —  7 1  7-  —  77  1  - 
b_ts.I-OO00OOCJ0O0O0C< 



t   7  r  7i  77  —  -7  i-  / :  — . 

K  00  OS  OS  35   35  35  77.   r-   3! 



16.0 

).  161 
.  L62 

.  Hit 
.  1 65 
.  166 
.  1(17 
.  His 
.  L69 
.  170 
.  171 

77  -7  '7  7S  r~  X   77.  —  71  M 

i,  r,  i,  r^  r--  t-  t>  X  x  X 

—  i  7  77   X    77.  77  —  7  1  —  •  7 
X   X   X    X    X    r.   77  77-  77.  77. 

C  N  /   7  -  7|  CO  '7    C  1^ 
Zl  —  —  7i  71  ~]  fi  fi  fl  fl 

OlONW^iOOt"  X  OS 



- 

—  71  77  —  >7  V7  t~  77.  77  — 

x>  oo  oo  oo  oo  oc  x  x  r;  ~ 

:i-7-t:i_-'/  3S  Q CM  " _" 
—  —  —  —  —  —  Zl  cm  CM  fi 

—  '7   '7    X    77.  777  —  77  —  1 7 
CM  CM  71  71  ~1  71  71  71  71  71 

r 

— 
— 

ts.00Oi— iCMeOTHiacDt— 
I-  1^  X   X   X   X    X    X   X   X 

—   ~  —  71  77  —  '7  1  -  X   ~ 
S    ~   77.  77-  77.  77-  77.  77.  77.  77. 

77  —  71  —  i7  CC  r;   X    77  — 
CM  CM  7"l  LN  71  71  71  71  7  1  7  1 

—   £   1  -    /    7    —   7 1  77 

717171 

71  71  71  71  71  71  71  71  71  71 

o 

L3.0 

.  -  CC  X  CJ5  O  --<  05  CO  "#  U5 

X)  00  GO  00  OS  03  D-  77.  77.  77. 

, , , 1   , 1 1   T— 1   1 1   1 1   1 1 

1-   r    r.  77  —  71  77  i7  7T  I^ 
7    7    77.  ~~77~7;~7r: 
71  71  71  71  71  71  71 

/    ~  -    ~    :  i  -  7   —     7    7    /    7. 
—  — 

71  71  71  7  1  71  71  71  71  71  71 

77—  71  —  .7   77  r-  77.  77   — 
7171717171-1717. 
717171  71  71  71  71  71  71  71 

o 

L2.0 

co  -ct<  to  I—  oc  sohnco 

OSOJOSOiOJOSOCOC 

r.  t.  x.  r_ cm  cm  cm  cm 

d 

.7   -T  I~  X  ~ ■  ~  —  77  —  it 

77777777— 

71  71  71  71  71  71  71  71  71  7  1 

CC  t-  X   77   —  71  77   7'   77  t- 
71  71  71  71  7i 

71  71  71  71  71  71  71  71  "  l  71 

/  r.  r            — 

CM  CM  C 

11.0 

—  2"  ~  '.3  S  tr  £  S3  —  ~ 

7~>  7~i  7~i  CM  71  CM  CN  71  CM  71 

7 '7  77  I-   »C3»i-l( 

71  71  71 

71  71  71  71  71  71  71  7)  71  71 

-'77    /    77-7"      " 
7  1  7  1  7  1  7  1  "7   :7  77  77  77  77 
71  71  71  71  71  71  71  71  7  1  71 

7  1-   /:   7-71  7"  '7   77  1- 



71  71  71  7l  71  71  71  71  71  71 

o 

10. 0 

3SOCMC0'*«0«0t-000: 

—  71  77  —  '7  77  I  ~  77.   77  — 

71  71  71  71  71  71  71  7  i 

71  71717  1  71  71  71  71  71  71 

7 1  77  —  77   l  -   '    ~    77    -  l  " " 

-    —  —  — 

71  71  71  7"l  7'l  71  717  17171 

—  ■-      "     /      7     7    -    - 

—  —  —  —  —  >  7  ■  7  '  7  ■  7  '  7 
~l7l7!7l717l7i7i7171 

- 

9.0 

n  x  c  -  :i  r  -r  '"  cc  t- 

71  71  7)  CM  71  CM  71  71 

71  71  71  71  71  71  71  71  71  71 

77.   77   —  7  17 71-/    77. 

r*l  ~^  -^  --  -^  -^  -^    --  -^   -- 

71  7"l  71  71  71  71  71  71  7"l  71 

C   —   "  1  —   •  7    7   1  -    f     7-    — 


71  71  71  71  71  71  7  1  7  I  7  I  7  1 

7 1  7-7  —  77  !  -   X    77.  — 

"      7'7'7'7'7'C    7     7 
-17  17  17  1717  1717  17171 

© 

- 

i7  CC   X   77-  77-  —  71  77  —  '7 
71  71  71  71  77  CO  71  CO  77  77 
71  71  71  71  71  71  77  71  75  71 

NOOroOHNMlOCON 

-^  -^  --  —  — 

71  71  71  71  71  71  71  71  71  71 

/    77.  77   7  l  :  7   —  ■  7    7    /    7. 
—  —  '7'7»7'7'7'7'7'7 
7l  71  7l  71  71  71  71  71  71  7l 

CM  7~l   ~l   ~i  fl   ~l  CM   ~1  71  7  1 

«-s 

7.0 

77  -"  7C  t-  X   77.  77  —  7!  77 

71  CM  71  71  71  71  71  71  71  71 

d 

-■7  CN  "/   77-  77  7  i  -  7    — 

—  — 7.  i7   '7  '7 

71  71  71  71  71  71  71  71  71  71 

.  7    7  1 '  7.  C  —  7  1  7  7  •  7  77 
7"l  7"l  71  CM  f"i  ~1  CM  71  71  7"l 

1  -    /    ~    —    "             —     7    I  -    / 
CC    C   77  1  -  1  -  I  -  I  -  1  - 
-17  1-1717171-17' 

o 

•- 

;-::-77i-/  r.  77 

—    —    — *-    —    —    — ■*"   Tt^  — fl   ^*  LO 

717171717171717171  71 

71  77  —  '7  7Z  I—  X   ~  —  71 

'7   '7   '7   '7   '7   1 7   '7   77   '_   '7 
71  71  71  71  71  71  71  71  71  71 

7-.7I-  /  7.  :-"  - 

77    77    77    77    77    77    1  ~  1  -  1  -  1  - 
71  71  71  71  71  71  71  71  71  71 

■     7    1  -  77.  77  —    :      - 

[-i-i-i-x   f   x   x   x   x 

71  71  71  71  71  71  71  71  71  71 

■- 
3 

55 

ft, 

2 
ft 


- 
- 
< 


77 

id 


77 
77 


o 

71 


77 
77 


3 

— 


X  77.  —  71  7"  —  .7  77  I-  X 
—  —  i-  t7  »7  '7  »7  '7  '7  i7 
7  1  71  71  71  71  71  71  71  71  71 


777  —  71  77  —  '  7  CC  X  77.  77 
7C  CC  CC  CC  77  77  CC  77  CC  !~ 
71  71  71  71  71  71  71  71  71  71 


—  71  "7  '7  77  I-  X  77.  —  71 
I~  I-  I-  r-  I-  t-  I-  I  -  X  X 
71  71  71  71  71  71  71  71  71  7  1 


-    /    7     7    "        -    — 
X    /     X    X    X    X    77  77    77  77 

-      -I  71  71  71  71  71 


O  r-  05  O  i-i  CM  co  -r1  i-o  cc 

1-7  lO  lO  CD  CC  7C  7C  CC  CC  CD 
71  71  71  71  71  71  71  71  CM  CM 


OOTTCCC  —  71  77  —  'CI-  X 
OCCt-t-l-Nl-l-l-l- 
71  71  71  71  71  71  71  71  71  71 


77.  77  —  " '  —  '  7  77  I  -  77.  77 
t- 00  CO  00  00CX  f  X  X  - 
71  71  71  71  71  71  71  71  71  71 


—  OICO-OCCI-XC—    " 
77777777C77 

71    717I7I71717170707O 


—  ■  7  1-  X  7CJ7D— 'O370-* 
7  CC  C  77  X  1-  t^  t^  t^  t^ 
71  71  71  71  CM  CM  CM  CM  CM  CM 


CD  t-  CO  C35  O  --  01  -?  ■  7  cC 
(-NNNW3C/  X  77  X 
71  71  71  71  71  71  71  71  71  71 


1  -  X  77.  —  71  77  —  ■  "  1  -  X 
X  X  X  7  7  7  7  7  77  77. 

71  71  71  71  71  71  71  71  71  71 


_      _    .  _ 


71  77  i-0  C7  t^  CO  CC>  CD  —  71 
NNNt-t-NNK'/.X 
7-1  7'  71  71  71  71  71  71  71  71 


—  '7  77  I-  X   77-  77  71  '7   — 
X   X   X   X   X  X   77.  D.  D.  D. 

71  71  71  71  71  71  71  71  71  71 


'7  77  r-  77.  77  —  71  77  '7  77 
77  77.  77.  77.  77  77  77  77  77  77 
71  71  71  71   .7   CO  COCO  COCO 


—   —   S  ZLIIIZ  £'"  {. 


77  —  c 10  CC  t>  X   77  77 

X    X    X    X    X    X    X.  X   X  77 

71  71  71  71  71  71  71  71  71  71 


71  77  —  i0   D  1-  X  77  —  71 

D.  D.  77  77  77  77  77.  77  77   77 
71  CM  71  71  71  71  71  70  77  77 


—  '  7  !_;    /   7  7  —  CO  — 


'7  7  1-7.   7  —    I      —     '    Z 
71  71  71  71  71  71 


X  77.  —  71  77  -«  '7  CD  t^  CO 
X  X  ~  Z~-  D.  77  77  3!  77-  77 
71  771  71  71  71  71  CN  CM  CM  CM 


O  —  71  77  —  ■  7  CD    X    ~   77 

77777777777-;OCD77777  — 
77  TO  77   70  77  70  TO   TO   TO  TO 


—  7  1   77   ■  7    7C   1  -    X    —    —   7  I 

—  n  —  7777  7777  77 


■-    —  .7   I  -    X    7    7    "  - 

71  71  71  71  71  71   ■  " 

-^-   -^    ---    -^-    -^   — /-    -^-    --    -^   ^r- 


: 7  i*  lO  CD  t^  CO  OT5  ©         —  71  TO  -f  17  CD  I-  X  77-  CD         —  71  TO  "• 

n  ~  zl.  **■  ~z  n  if  zn  rs  'i    l*-  ■'  • "'  •  ~-  •  ~-  —     -d-  d  cc  cc  cc  -d'  cc  cc  d  i  - 


—   -1      "    —  7    7 


I-  t-  t-  t-  I-  t-  t-  r-  t-  x 
-  — —  — 


RESEARCH  METHODS  IX  STUDY  OF  FOREST  ENVIRONMENT.      189 


•  oc 
■  c 

•  S 


.  IC        C  N  X  Ol  C  "  fl  i1  C  ; 

■  O        O  O  O  O  ~^  ' — i  ' — '  ' — i  i— t  ' — 

■  o      oooooooooo 


■  <n      co  ■*  to  f-  oo  o>  o  <n  eo  ■<* 

■  — I         —  r—  . —  OI  OI  0*1  !M 

■  O       OOOOOOOOOO 


■0 


■  O         ,—  —  —  —i-h,—  ,-j,-|t-icN         N  C-l  Ol  W  N  N  (N  K  CO  W 
•  O       ©OOOOOOQOO       C  O  C  O  O  O  ooco 


ilO         N00O)OHCtCQ>0£N         '/.  32  O  N  CO  'f  O  ffl  N  X         Oi  O  — '  CO  -t"  '0  C3  X   3:  — 


re  —  ■-  i-  x  —  o  —  ci 


-r  >o  eO  t-  oc  ~  O  —  o  i  — 


•  o  co  i-  o.  —  oi  co  —  >o  50 

01  01  01  0  1  CO  OO  OO  00  CO  OO 


I-    /    JlHCqcOOONX 

cococo-r-ti-r-f-iTtH^-ti 
oooooooooo 


g  . 

c  — 

'  s  ~ 

r<  =. 

CO  — 

0 

•~.  — 

— -^  — 

jo  ft 


5o 


*  ri        — 

^    - 

v.    - 


c 


c 
= 


r- 


1  0 

-:i-i:Ti-/r.r- 

oi  co  —  '0  co  i  -  x  —  o  oi 
oi  oi  oi  oi  oi  oi  oi  oi  co  co 
oooooooooo 

co  —  i  o  i  -  o  o  —  o  i  co  — 

*^cocococO't-t"T"~"r 

oooooooooo 

lOtDNO>QHCO^<IOCO 

-r  -r  -r  -t  iO  iO  »C  lO  ic  »0 

o  ooooooooo 

— 

o.  ~  oi  co  -r  i".  -_o  i  -  /   pa 

-CICICICICICICIMM 

oooooooooo 

—  —  OI  CO  —  '--    -    /  OSO 
CO0OCOCO0OCO0O0C0O  — 

—   OI   CO   '  0    1-    A    0>CrHN 

r  —  -r  -!"  -r  i0  '0  i0 

OOOOOOOOOO 

CO  •"■*<  icO  h-  CO  Oi  — i  CM  CO  ** 

■  C  '  0  LO  115  LO  K)  ffl  !D  (C  to 
C  ©  O  O  O  ©  0*0  S  o 

— 

M 

I-  A o  —  0  1  oi  co  —  i-.  -- 
o  i  o  i  co  co  co  co  co  co  co  co 

X  ~  O  —  o  i  co  —  EC  i  -  X 

co  co  *-*■  - —  ~~  - —  - r  - •*  — **  — ■ 

O-    O  —  CO  ■  0    '3   1  -    •    35  O 

•"  '*  O  O  '"  O  i"  i"  it  ■« 

-H  04  CO  'O  C  N  Q  C  H  W 
50  EC  EC  EC  EC  EC  SI^Nt^ 
OOOOOOOOOO 

- 

•  -.  ■-  s  3-  oo  o  —  oi  co  -r 
co  co  co  co  — *  —  —  — .  —  — 


■-  i  -  y  —  o  —  o  i  —  •  -  ■ -0 
—  —  —  —  'O'O'O'O'OiO 


i -  s  31  —<  co  -<r  >o  o  i ~  x 

'7  'O  O  C  C  C  C   EC   —  53 


35  O  —  CO  -f  'O  1^  X  35  O 

=  1^  lr  '^  Ur  Ur  Ir  !_:  !^  ^1 


oi  co  '0  -c  i-  y  3:  c;  —  oi 

—  —  —  —  ■-■  -t*  -r  io  io  >o 


—  ■  o  j:  i  -  r  r.  C;  oi  co  -ri 

'0  '0  '0  '0  '0  '0  EC   EC   ~  "» 


-  -  i  -  ~  —  o i  co  -r  >o  x 
-jo  -s.  —  ~  i  -  i  -  r~-  r^  r^  t^ 
OOOOOOOOOO 


t^  0G  (35  i-l  CN  CO  'O'  CC  Is-  00 

i  -  i  ~  i  -  y   x   s   /   a  /   s 

OOOOOOOOOO 


~  —  co  -*•  >o  -^r  i  -  /:  3~.  c:       oi  co  -r  >o  -jo  r~  x  o  —  oi 


»_0  » 0  <  0  »  0   'Jt    i  O  '  O  '  O  i  o   ~ 


=  =  ■=•=  =  -=  = 


oo  o 


CO  ■*  i.O  N  ffi  O  --H  OI  CO  t 

t^  r^  i-~  t^  i  -  s  /  s  j   s 

OOOOOOOOOO 


UO  «0  t~-  35  C  — ICO<tl<iQCC 

/  s  j  /  35  noiccs  o 

oooooooC'CO 


r  -  —  o i  co  -r  •  o  ;i-/ 

1 0   »_0   *J0  'JO   »   53  53  »  »  EO 


O  — i  OI  CO  -T  l0  53   V.  35  O 
l-l-l^t~t~t^t-~l^t-   y 

oooooo  —  —  ~  —  ~^.  — 


—  OICOlONXCSC'HN 
/    f    /    s    /    /    X  35  Oi  35 

ooooccocoo 


co  co  "O  r>  a:  oj  t-i  oi  co  -t 

3505050535050000 
OOOOOOt— It- li— IH 


CO 


01 


CN3>C  —  OlC0-tiO3 

ec  =  50  Lr  Lr  i  -  i-  '~  r~  r~ 


S.  ~   00  —  OI  CO  tf  53  t^  S. 
t^l^'A    A    A    A    A    A    A    £. 


3".   00  —  CO  f-O  3  h-  35  00 

X  00  35  35  35  35  35  Oi  05  O 
OOOOOOOOOO*— 


—  01  CO  iO  -iNOiOiHCN 

OOOOOOOt-Ht— It— I 


-+  >-0  I-  A   O-  00  —  OI  CO  -V 

t~  t~- 1- 1- 1»  oo  oc  x  x  x 

3333330000 


CO  t-  OC   3J  O  «-»  CM  •>#  tO  <C 

A.  A.  X  A_  35  31  35  35  O-  35 

oooooooooooo 


t^XSiT-iCNCO-flOt^CC 

O-O.  350000000 

©  ©  O  t-H  t— Irii — I  1-H  1 — it— 


05  O  t—  CO  Tf  LO  CO  CC  05  o 

Ot— It—  — H— ^t-H^Ht—  t— lOl 


r: 
W 

PS 

< 


oi 

OI  CO  LO  53  O  X  O.  O  t-h  CM 

v  x  r.  y  a :  y  y :  35  35  oi 

OOOOOOOOOO 

hh  ic  3  N  X  Oi  3  OI  CO  -f 

O   O    35   35  35    35   O    O    O    — 

LO  5C  N  3:  3  tH  M  CO  lO  O 

NcCiOiT- icaco-^ot^oo 

0 

O  —  CO  "t  i0  3  N  X   350 

3.   3-  35  O-  35  35  OI  35  35  O 

NM*«0(D>00OHN 

CO-tuONXOOT-tCOt 

—   — It— 1— It— li— ICMCMOMCM 

LOCNOCClOTHCOTtilO 
C<I(NCNC-10N1COCOCOICOCO 

o 

-r 
01 


y  3-  —  o  1  co  t  ■*  53  t--  x 

o-  o-  ooooooooo 

OO' 1 !  — 


O  — '  01  CO  -ti  i-O  53   A  OI  O 

—  — , ^,^-M 


rt  OI  CO  lO  CO  N  X   ~   —  0  I 
CM  CM  OI  OI  OI  OI  CM  CM  CO  CO 


CO^iONOOKOlHMCO 

coeocococococo'cti'cf'ti 


Wet 

bulb. 

rH  01  CO  —  ■"  53  I  -  X  05  CO 

-f'  -r  -r*  -t-  — ■  —■'  -t>  -t-  -t ■'  ■  0' 

— f  -?*  —f  —  *T  "^  "^  "T  ~ t*  —H 

H  01  CO  -t  C  CO  N  X  S>  3 
iO  LO  »0  i0  iO  i-0  'O  i0  10  53 

^^H    ^J*    *I^     ^1^     ^J^     ^^    ^7^     ^T*      "  J       ^T* 

rt  CN  CO  ■*  LO  -O  N  X  O!  O 
cd  O  CO  O  co'  co  CO  CO  coV 

-   f '     ^T^      '  J '     ^OT-       '  J '      ■■  J  *       '  J       *~^"     "^*-     **^ 

HCNMfOCONOCOlO 

t^  i  -  t^  t-^  1^  1-^  r^  r^  t^  x 

^Ji    ^cy^    ^T^    ^J      ^4^    ^T^     *-*'    ^jp    "^T^    ^^ 

190  BULLETIN   1059.    U.    S.   DEPARTMENT    OE   AGRICULTURE. 


— 

^ 

-" 

— 

a 

: 

— 

• 

* 

. 

° 

cr 

s 

co 

•  S 

o 

x 

- 

«o 

r 

V 

k 

— 

a, 

z 

^ 

^ 

BO 

0 

<SJ 

s 

*C£ 

S 

•   !» 

— 

M 

— 
-*• 

»-| 

- 

»-* 

|3 

pq 

V 

Fh 

s 

w 

0 

£ 

09 

- 

-C 

t 

t>a 

'  S 

^ 

<W 

fc. 

s 

co 

co 

w 

&. 

V. 

C 

&, 

,e 

- 


r. 

-: 
M 
- 


P3 


7) 

pa 


c 
77 


C 


t- 


71 


■^zz 


pa 

— 

42.0 

—  • —  - — 1  i — 1  i —  C<1  C)  71  CI  CM 

ll.d 

C} 

- 

7i 

y% 

— 

g 

I-  X 

^  i—l 

22  JJ 

•M  X  i~  -^  t^OJO  —  rc- 
Ci  t  i  ?)  *i  ri  ?i  rt  ^  ro  ro 

I  -  — -   ~  —  7 1  —  >  C    —    S    ~. 


—  '7  —  IC  t~  A   ~  —  7> 


irat»oocj»©cNco-*cot-       /.  c  -  m  r:  '*  '"  -r  z  c 
C^eMCNCNCOCOOOCOCOCO        re  ?C  -r  —  -"■-*■- ' -  —  —  t~ 


re  *"  --C  l>»  OC  —  — '  71  -r  *-t 
pa  co  co  co  co  ^*  ^*  ^  ^  ^ 


>t-a>Oi-HCO-<*»Or--OC 

1  -r  -t*  >7  i7  ic  ua  '"  wa  '* 


c  te  x  c»  o  cm  co 

-r  -r  —  —  >-  >-  ■- 


■  -  [  -  X  35  —  7>  77  •  ~  \Z 


—  7i  :-  ~ .-  —  t-  y  —  = 
s    S_    f    s    f    /   s   X  s   ~ 


7  i   -  - 


Hi  --  --  ~  *  ~  ~  ~-  — 


RESEARCH  METHODS  IN  STUDY  OE  FOREST  ENVIRONMENT.      191 


-3 

3 


C 
O 


8« 

C*> 


o 

B 

oo 

cj 


c» 
J- 

CO 

ao 
tt 
A, 

© 

a, 


w 

pq 
< 


q 


w  t  >r:  ^  r-  a  c  -•  n  -r 

oioiaoiaaoocc 

_<_H,H    —    __OJCM0101 


^CCXOSCOCMCOrf'iOl"- 

CMCMCMCM'cMCM'oi'cMCMCM 


CM  CM  Cl  CM  <0J  CM  CM  Cl  Cl  Cl 


CM  co  -r  co  i  -  os  CO  —  ?;•* 
£  co  co  co  co  co -r -t-  —  IT 


q 

CO 


§?4?3S!J?fe82c3  CM  W*<DNOOOihNWIO  tON»C(NMt<mNy  O-JMnim^nn^-,*, 

«?!««as«aS5  SSSSSSSSSS  sSISSieliafl  I3333£i3£g 

c  

o  *         


3 


q 


-  x  *  ;  -  ^  »  l-  •-  / 
;t".oo;c-:!Ncmcm'cmcmcioi 

O)  N  W  M  M  N  »4  ?l  M  :i 


os  co  cm  co  —  co  r^  x  os  — 


cm  co  '•  co  x  cc  co  cm  co  ~r 
—  —  —  _f.  — —  —  i  —  .  —  i—  i- 

Ol  (M  W  !N  (M  IN  C>1  N  !N  f  1 


CNXC  —  CO  Tt*  lO  IC-  OC 

ic  m  ic  to  co  co  co  co  co  co 

N  N  N  ?)  N  N  N  M  «  N 


q 
co' 


q 
cm' 


c*> 


.© 

«o 
«o 

^    cm 

*    -" 

a  <- 

co      o 

■8  -■ 

s 

•<s> 


-r 

a 

P 

» 


q 
o 


q 
OS 


q 


q 


q 
co' 


q 


q 


q 
cm 


q 
© 


3 


3SSSt:0025'-,<>>eo»occ 

CMClCMOlOlcc-ocoioo,^-. 
CM  CM  Cl  Cl  CM  CM  Cl  Cl  CM  CM 


cm  ~r  i-  ci-~  c-of 
co  cc  cc  co  cc  ~o  — r  -r  —  — 
CM  CM  01  Cl  Cl  CN  CI  CI  01  01 


t~-  V  CO  —  CM  -r  >0  CO  t"~  eft 

WK-f-f  f  -fr-t 
CI  CI  CI  CI  ri  ?1  ^1  ^  ^  ^1 


'"  —  X  r.  ;  ri^tici^ 

—  —   —   —  '  0   I  ~   >0   I  *   i*  1C 
CM  CI  CI  CI  CI  CI  CM  CM  CM  CM 


~  —  cc  -f  co  r~-  x  co  — i  cm 

'0»0<OiOiO»0'OCOCCCC 
Cl  CM  CM  CM  CM  CM  CI  CM  CI  CM 


^t'lOCCXOS— <CMCO»OCO 

co  co  co  co  co  t-  r^  t-  t^  t^ 

CM  CM  CM  CI  CI  CI  CM  CM  CM  CM 


X  o.  —  cj  -r  i-  co  x  os  co  cm  co  -t<  co  r-  os  co  —  co  -f 
igi/JCOcOcOcOCOcOcoi-.  I  -  t-  t  -  1^  I-  P.  x  A  j  A 
CI  CI  CI  CI  Cl  CI  Cl  CI  CM  CM         Cl  CM  CM  CM  CI  CI  CI  CI  CI  CI 


CO  CM  cc  —  .  o  i  -  a  ~   —  ci 
-r-^>  —  -*  —  -"-*  —  i-, - 

CM  CM  CM  CM  Cl  CM  CM  Cl  Cl  Cl 


CC-*CCI-X    C—  CICC'C 

>~  i.c  i-  ic  •-  cc  co  cr  co  co 

CMCMCMCMCMCMCMCMCMCl 


—  i  ^  oi  co  cm  cc  -r  co  i  -  y 
co  co  co  i  —  i^- 1^- 1^  r~-  t^  r-- 

CM  CM  C)  Cl  CM  C)  CJ  CM  CM  CM 


CO  -h  Cl  -f  •-  1^  A  Oi  — l  CM 
»  00  CX  r  X  X  X  X  35  Cft 
CM  CM  CM  CM  CJ  CM  Cl  Cl  CM  CM 


A  —  —  c  i  :c  re  ■ ~  i  -  A  ~ 
~ *  •"  <c  i"  i*;  i-  i"  i-  i-  -* 
Cl  Cl  Cl  Cl  CM  CM  CM  Cl  C  i  S  I 


—  Cl  —  i-  CO  A  ~.  ~  —  c- 
Cp  CO  CO  CO  CO  CO  CO  I  ~  I  -  I  - 
Cl  Cl  Cl  Cl  Cl  Cl  CM  CM  Cl  CM 


>*iOf.oe 

i  - 1  - 1- 1-  _. 

Cl  Cl  Cl  Cl  CM  CM  CM  Cl  01  CC 


x.  —  —  —  "T"  '" 
?'  X    i     j     /    j 


i  -  x  cs  —  ci  —  >c  co  y  35 

X    X    X    35  OS  Co  C3J  OS  OS  OS 
CM  CM  CM  CM  CM  CM  CM  C-l  Cl  Cl 


co   x  eft  CO  —  cc  *r  »":  co  A 

lOlOlCCOCOCDCOCOCOCC 
Cl  Cl  Cl  Cl  Cl  Cl  Cl  CM  ^1  >"M 


35  ~  c  I  CC  —  i "   CO  I  -   A    CO 
C0I-I-I~I-|-|-|,|-    / 

ci  ci  ci  ci  ci  ci  ci  ci  ci  ci 


—  ci  —  ■-  i  -  a  ~  —  ci  :: 
Ay    j    j    /   /   j    o.  ojc> 

Cl  CM  CM  CM  CM  Cl  Cl  Cl  Cl  Cl 


•c  co  t^  Oi  co  ci  c  -r  co  t^ 

OC  OC  C.  OC  CO  CO  CO  CO  CO  CO 
Cl  Cl  Cl  Cl  CC  CC  CC  CC  CO  CO 


"f'-OI-X  05  — ■  CM  CC  —  ic 
-0  CO  T  ■-  co  I  -  I  -  i  -  r  -  i  - 
CM  CM  CM  CM  Cl  CM  CM  Cl  CM  Cl 


CO  I  -   ~    —   —  -  -   —  .  -   CO    A 

h-  r-.  in.  a  /   j    /  /   ,  , 

Cl  Cl  Cl  Cl  Cl  Cl  Cl  Cl  Cl  Cl 


OiOCMCO'CcOt^OlCO  — 
A  3".  05050505O5O5CC 
Cl  Cl  Cl  Cl  Cl  Cl  Cl  ^1  CO  *^ 


co  ^t;  io  t-  x  co  — i  cm  -r  ic 

COCCCOCOCOCOCCCOCOCO 


—  CO  —  i-   CO    A    35  0CMCO 

i^  r-  i-  i-  i- i -  i-  7  >    / 
CM  CM  Cl  Cl  Cl  Cl  Cl  ci  01  ci 


-r  <~.  i  -  a  z-  —  ci  --  -f  co 
'   >   x  /■  x  OS  co  co  an  oi 

CMCMCICIC1C1CICICICI 


io;  a  r;  —  cc  —  i_c  i_;  /.  oc 

Cl  Cl  c*  ^o  CO  CO  ?~  c^  c^  ^ 


—  Cl  CO  >~  CO   X  OS  CO  CM  CO 

—  —  —  —  —'  —  —  Cl  Cl  Cl 
CCCCCCCCCCCCCOCOCOCO 


~  —  C I  CO  —  CO  I  -  A  —  — 
'  -  A  /  /  /  /  /  X  OC  35 
CM  CM  Cl  Cl  Cl  Cl  Cl  Cl  Cl  "I 


Cl  CC  i  C  O  I  -  O  CO  —  C  I  — 
35O5O5O5O5O5C0C0C000 
Cl  Cl  Cl  Cl  Cl  Cl  CO  CO  CO  CO 


■_0   CO    A    O.  —  C I  CO  1 0  CO  I— 

cccocococccocoTCcocc 


OS  CO  —  CO  -r  CO  I  -  X  CO  — i 
—•  Cl  Cl  Cl  Cl  Cl  CM  CM  CO  CO 

cocococococococococo 


'--•_  —  C I  —  i  0  CO  A  ~- 
»  00  OS  CB  31  O.  35  OS  OS  OS 
CM  CM  Cl  Cl  Cl  Cl  CM  CM  Cl  Cl 


CO  —  CO  -"  ■  -  I  -  A   OC  CO  Cl 

cocococooocoooco  —  — 
co  co  co  co  co  r~ 


I     --^     -1^     -N-«     -v— 


co  —  co  t~  as  c  —  co  —  i  -. 

—  —  —  —  —  CM  Cl  CM  Cl  Cl 

CO  CO  CO  CO  CO  CO  c~  "*~  "*~  *^" 


i  -  a  oc  —  c i  -*■  to  co  x  as 

Cl  Cl  Cl  CO  CO  CO  CO  CO  CO  CO 

cocococococococococo 


'O  r-  a  oc  co  ci  c-  —  co  i~ 

oooooooooc 

CMCMCMCICOCOCOCOCOC* 


x  as  —  ci  co  'O  co  t^  x  co 
cz  z^  ^~  CO  -^  -^  ^-  -s-  -^  ;c- 


—  Cl  -t<  O  l~  X  OS  ^-  CM  CO 
Cl  Cl  Cl  Oil  CM  Cl  Cl  CO  CO  CO 

cocococococococococo 


>c  co  t-~  as  co  ci  co  -v  co  t>. 

cocococo-^-f-r-f-f-r 
cocococococococococo 


2  ^-  =  Lo  £.  °  —  cm  -*  '*o 

cocococococococococo 
o  


C0t^-~.  CO  —  CO  -I-  ".0  CO  X 
•—— '  —  CMC1C1CMCMCMCM 

cocococococococococo 


OONCOlOONCSOH 

Cl  CO  CO  CO  CO  CO  CO  CO  -r  -*■ 
COCOCOCOCOCOCOCOCOCO 


CO  "*  "O  1^  X   CO  —  Cl  -t1  «-0 

-v  ~*f>  ~r  -fi  ~r  uc  io  io  i.o  >o 

cocococococococococo 


—  co  -r  '0  co  x  as  co  —  co 

i — I i—l  —  —  , — 'CM'NICM 

cocococococococof-co 


•*'CI-   X   OS^-CMCO  —  CO 
Cl  CM  Cl  Cl  Cl  CO  CO  CO  CO  CO 

COCOCOCOCOCOCOCOCOCO 


I  -  A    CO  —  CO  -r  <-0  Is-  A  as 

coco  —  -r-T-r-T  —  -r-f 
cocococococococococo 


-;  CM  CO  .  0  CO   X  OS  CO  CM  CO 

>  .  '  .  <~  '~.  IC  iC  •-.  CO  COCO 
cocococococococococo 


OS  —  CMco— ■  On  /  3S  — 
—  Cl  Cl  CM  CM  CM  CM  Cl  CM  CO 

COCOCOCOCOCOCOCOC<ccv" 


ci  co  i-o  co  i^  a.  c  —  cm  -r 
cccccccoccco  —  —  -r  T 
cocococococococococo 


1  '■  —  x  a.  —  c  i  co  ■  o  co  r^ 

—  —   T  'O  >0  'O  lO   >C  IC 

cocococococococococo 


as  O  —  CO  —  co  I  -  x  oh 

'  1  CO  CO  CO  CO  CO  CO  CO  t^-  1^ 

cocococococococococo 


tc  OS  C  -  Cl  H  iC  O  N  OS 

CMCICOCOCOCOCOCOCOCO 

cocococococococococo 


—  —  CO  —  iCNZCOM 

t  -1"  -t  -t  T  -f  f  f  IO  io 
CO  CO'  CO  CO  CO  CO  CO  CO  CO  CO 


co-rcDt^aso  —  cc-rio 

to  tc  »0  'O  »o  CO  CC  CO  co  CO 

cocococococococococo 


t—  x  as  —  c-i  -r  io  o  x  cs 
co  co  cc  t^- 1^  t—  t—  r^  n-  i>. 

cocococococococococo 


cocococo-r  —  -r  -r  •&  -r 

COCOCOCOCOCOCOCO'COCO' 


x  as  —  cm  co  >-o  co  t—  x  co 

"T  -f  tO  i-C  lO  '0  "0  iO  L"^  CO 

CO  CO  CO'  CO  CO  CO  CO  CO  CO  co 


—  cm  -f  »o  i~  x  a.  —  oi  pq 

CC  CO  CC  CC  CC  CC  CO  t^  t^-  N 
COCOCOCOCOCOCOCO~0'-^ 


'O  CO  t^  OS  CO  CM  CO  -^f  CC  t^. 
1^  t^  1^  1~  X    A     A     J     f     / 

cocococococococococo 


~ I  CM  00  Tf  UO  CO  t-  X  OS  CO        — '  CM  CO  Tfl  IO  CO  r-' 

LLLLLiii\L^      SSc5r;^i=o=c=c  = 


oo  co  q       i—  ci  co  ~r  i  o  co  i  -  a  a.  oo 
ic  l.o  65  io  t~  io  re  i~  i"o  io 


—  c  i  co  —  i  o  co  i  -  a  as  co 

» 0  tO  ' 0  » O  1 0  '  0  '0  i  0  '  o  >"o 


192  BULLETIN    1050.    U.    S.   DEPARTMENT   OF.  AGRICULTURE. 


— 
- 
- 

- 
— 
— 

- 
: 
_ 


^ 


-c: 
— 


^ 


- 

I 

*c* 

eo 

"-C 

as 

— 

ft, 

- 

v^5 

_ 

• . 

CO 

<o 

(N 

i-=: 

lO 

o 

z 

"■ 

0 

-- 

00 

i-i 

— 

<5^ 

PO 

5- 

i-3 

- 

W 

= 

f- 

o 

M 

t. 

> 

e 

r* 

►© 

«0 

<u 

-c: 

e. 

= 

^. 

-^ 

■^ 

i 

eo 

oo 

c» 

s. 

ft, 

$~ 

o 

- 

>« 

- 
- 

- 


ec 


§22s33SilI  IliliiisSS  llllilllll  l§illl§ll§ 


i~rr.ce 
m  m  cp  i 

—  re  re 


^  5  re  re  re  re  re      cbcccc:::: 


re  "r  —  t-  ~  —  —  —  —  ■  -      1 92  2 
x  i  2  2  ^  5  S  J  S  5      —  —  - 


p-i  cn  eo  tjj  us  t«-  oo 


— 


i|§||gg|g  llglllllll   liillllsli   =E=l§rz==f 


re 
P0 


CM 

CO 


'_T  '-C  Ic  '_C  '-T  i  —  —  ii  —>OOOOOOOO0         O 


—  —  ~)  ~-  ,~  ot^aac       ci  re  —  to r—  0» O •-< eo "* 


_ scpooojo^e*      3<"Sft2gSgg§oc=      SoS^S^SSSS      °Se3cNc3e5e38eoec 

oooooooooo      O  O  O  CO  CO 1 """'  • .         


q 

— 

re 


—  —  -i ec  t~-  ec  ce  —       c !  2r 

at  ~  33  ce  r.  *  J  ;  J  "      o  o 


rc'rcrcrc  ^5^^=       S  S  3  S  §  S  ?!  3  g  §       JJ  fj  |  g  H  Lc  «  2  «  = 


SSSSSoooa       =:2"_-£--n?l       elc-iricicirircforcre 


—        -  <~  s  —  —  ■? •-[-/■ 


re 
ro 


C3 


.rr-  x  33  c?iro-* 
re  CC  ~  C:  —  - —  i i 


L:  L:      _  S-i  fi  fi  ci  51  ci  n  fi       co  eo  eo  eo  eo  eo  ec 


i  i/j  ec  y  ce  —  Neons os 

—  ■  e  ■  e  ■  e  '  e  i  e 


(N 


SSS5S'§3222    8§8is!l238B    ^zz^'iiztl    gSSSjSSSSSS 


_i  m  t»  ii5  co  oo  as  o i-i  eo      -#  >e  r»  x  ~  —  ~>  ~~-  ~  •'■       —  '-  ~  S  rJ  PS  "T  -S  '-  -^       —  ~  ?J  ~  '.;  !z  .-C  .--■  r~  .r ' 
cn  cm  e'i  r'l  fi  ~i  ei  re  re  ce       re  re  re  re  re  -r« "'"'"'•'•  '^  '_:  L^       :;;:;;;;_- 

i    _    _    . —    ^-    — -i    -^   »-H  i— H    ^   ^-1   —    • —    —    •"  ■"" "    '  —    ^    —    ^ 


C  r-  ?)  rc  ■*  C  n  /-  ~  -"       <M 
CMcererer^rererere^f      -^f 


CC 


—  —  — , -  , e  ■  -.  ■  -       loioioioeoeoeocoeoec       '£.  j£  !_:  _:  '_!  !_T  _Z  _I  _T  .£. 


cooooso^HeoTitmcooo      oiONra*;i-/ --      ei  r'  >-.  ■-  /  r.  =  -i  :-.  —       s  ;  -  s  _  —  re  —  >~.  \-  f 
—-  -~  r+  zz.  ~~  —  _  _h  3fi  ^       ^ici-i-:iCiO>OiOiOc5        CO  eO  ec  CO  CD  CD  t- 1«- 1~  t-       y_Z  \Z  [Z  L  L  L  L  L  L  — 


•«*<cOls-00Ci'-!<NeO'cl<co 
—  —  •*-•"*'"  '"  'C  >e  j- 


— 


i -  s  —  —  e i  -~  . e  ^  i  -  -       —  —  re  —  -.;  i  -  /.  ~  —  ri       —  ■-.■-/■  r.  —  c.i  re  '_;  i 
ioScdcdcocococococc      Pn^ni^nnxxx      LLLLLZ-Z-WI-Z- 


re 


N^csNaCHNrf      Lcoxr-.  cckc-tici-       /■  -.  —  ci  —  •;  z  r  r.  r       r_i  re  —  -r  >_-  r-  =  — 

ic  ic  ic  ic  ie  lococococo       CO  CD  CD  CD  t-  r^  t^  t»  t-  r»       r^  i-  /    /_  r_  /_  /_  /_  /_  £.        __.  _.  -_■  -•  -_  -_-  _  _  _  — 


ci 
ci 


o  M  w  ■+  ie  n  x  s>  o  m      ce-t<tci~y"~  —  cire-e       v:  i-  er.  c;  ci  r'  —  -i-/       ~  —  ci  —  >r  i-  /  c  —  ci 
cC  ec  ec  ec  ec  ec  ec  co  t—  r-       t-  t-  t^  i>  h-  X  X  X  X  X        x   /   s -  ~  ~.  ce  ~  r  ~  r.       —  ec  re  C  cc  C  C  ~ 


r  —  c_i  —  -_e  r_-  r  — .  —  ci 
c~i  ci  c"i  c"i  ci  c"i  ci  ci  ci  ci 


q 
c"i 


•ec  —  —  ei  re  >e  eo  t»  oo  O       >-i  cm  ■*  i.e  ec  y  ce  cc  — '  re       t'e^/c--!-':;        /■   r   ~  c  i  re  .  -  ec  r  -  ce  — 
eot^t^i^-i^.r>-t^.t»t-oo       eoc/r/.  zxzsc.e       ce  ce  ce  ce  re  re  re  re  cc  ec       c  re c  i 

i—l  i— 1>— i,— i-Hi— i^hi— t  ^-1  i — I         ^h^^Hi— li-Hi— iHr-—  — CICIC1CICICI         CICICICICICICICIClCI 


o 

CM 


;—  Z-  .-•  ;■?  "7  :;  ~t  '  -  =S  o°       a  O  ci  ^  *  c  n  7.  c.  -       c  i  re  ■  e  ■-  s  ce  rr  c  i  re  —        c  i-  /   ce  —  re  —  .  e  i  -  / 
t~  Is-  r»  oo  or.  »  oo  oo  oo  oo      x  os  oe  cs  ce  ce  ce  ce  ce  re       re  re  re  re  re  re c  i  c  i  c .  c  i  - 1  - 1  - 1 

rH  i— rH  rH  —  —  —  —  ,— c  —  — CI  CICICICICICICICIClCI  C  I  C  I   C  1   C  1   C  I    C  I   C  I   C  I    "       " 


SIcoG-ooS-r-jMeOTHco      i^  er  re  —  - 1  —  i e  ec  i -  ce 
/;/./.  /  /  -  aoicsoi      ce  ce  re  re  re  re  re  re  re  re 

—  —  —  . —  „         , — |, —  ?l  CI  CI  CI  CI  CI  CI  CI 


re  —  re  —  ec  i  -  r  re  —  ci       -■:•:/    ■  -  ec 

—  —  ~—  ~—  —  _  _  ^i'^i'^i       —  j  -— '  j  —  j  —  i  —  j  — - —  ~"  -^  ~^  ^* 

Cl  CI  CI  CI  CI  CI  CI  CI  CI  CI         CI  CI  CI  ci  ci  ci  ci  ci  ci  ci 


— 

o 


— .  "!  n.  ~~,  '~  '-  <  •  s.  33  q       —  ci  re  —  w  to  i  -  /  ce  re 


x  ot  oc  ac  x  oo  x  x  x  ce 

~  —  — — ■  —  — 


3   ce  ce  ce  3S  ce  33 

—  —  —.  —  —  -^ ^  — . 


—  Cl 

re  r; 


q  i- 
c  >e 


i-  r  r. 


RESEARCH  METHODS  IX  STUDY  OF  FOREST  ENVIRONMENT.      193 


© 

00 


©  t^  ©  O  —i  00  Tf  ©  r~  © 
•*'*  —  »:  i~  i"  i*  i*  »-T  i-~ 
CMO10101O1O1O101O1O1 


q  ^  co  l0  ©  x  ©  —■  cm  tj< 
CM  CN  P»N«NNNN« 


octo)ffia>oiaioco 

CMCMCMCMCMCMCMCOCOCO 


© 


to  IC  iO  MS  iO  ©  ©  co  ©  CO 


ZC-Kf©NS!ON 

©i-r^-t^-r^t^t^r^ocoo 

Ol  Ol  Ol  Ol  Ol  CM  Ol  Ol  Ol  Ol 


xxxxxoioiOioia 

O104010IOI01CM010101 


XO>r-M'*i5DN»ON 
CMCMCOCOCOCOCOCOCOCO 


q 

© 


?1  ^  '^  C  N  O*.  O  Ol  ^t  Lt 

©  co,  co  cp  co  cp  lr~  t— 1~  t> 

M  ?1  N  ^1  N  ?1  N  M  N  N 


CC  X51-^'fiCNXO» 


OlttOiOSlfllOlCCO 


©•^-©©CM-t^loh^x© 

©©©--<  T-H  r-H  i— <  i— i  ,— i  CM 

coKm^c<3coccr:co™3 


q 


r^  r-  r--  t-  r-  r—  r~-  x  x  x 

Ol  ?1  M  ft  N  M  fl  M  M  M 


—  C  l>  J".  C  N  ?:  ^  *  N 
X  X  X  X  ©  ©  ©  ©  ©  © 
Ol  Ol  Ol  Ol  Ol  CM  Ol  CM  CM  CM 


XC'-W'tCNOS-N 

aoooooooHH 

CM  cc  cc  to  co  to  co  co  cc  co 


■^i^NXoiNmiotox 

r-lr-li- >  n  CM  CM  Ol  CM  Ol  Ol 

CCCCCOCOCOCOCOCCCOCO 


q 


t—  1^  X  X  X  X  X  X  x  © 

Ol  Ol  Ol  Ol  CM  Ol  CM  CM  CM  CM 


Ol  —  '".  t-  X  ©  ©  Ol  CC  «~ 

©  ©  soiaooooo 

CI  CM  CM  CM  CM  CM  TO  C.  CC  CO 


•c  /a-NTtcMsc 

©©©__  —  „_.—  CM 

cccccccccccocococcco 


(NCCOtOXOi-lcO*© 
CMO10401O1COC0C0C0CO 

COCOCCCOCCCOCOCOCOCO 


q 

CO 


©  r~  ©  ©  —  co  T  ©  r^  x> 

X     X     -T.   ©  ©  ©  ©  ©  ©  © 
CMOIOIOIOIOIOIOICMCM 


C.-Mt'CNXOHW 
©OOOOOOi—ir- ii- 1 
CM  co  to  PC  :  .  -■-  cc  CC  CO  00 


f  -c  t-  as  c  cm  co  in  r~-  oo 

,_H__-,-»CMCMCMCMCMCM 

CO  CO  CO  CO  CO  CO  CO  CO  CO  CO 


©■-<COt*«cC00O3>—  cm  -rj< 
COCOCOCOCOCOCO-'tl'"*-* 

cocccoa^cococococcco 


q 

CM 


—  "  ~  t  -  x  os  ©  i— i  co  ■*  ec 

©©©©©©©-—©© 

CMCMOICMCMCCCOCO    COCO 


r^  ©  ©  ci  co  '"  ©  x  ©  ~ 

©©, I r-    CM 

:.  cccctotctotocccc 


CM  ■*  iO  t^  X  ©  —  CC  IC  CO 
CMCMCMCMOICCCOOOCCCO 

cocococococococococo 


0C©rHCM-rfi©t^.©©Ol 
COCO^f-*-*'<*<TfTt<iOiO 

COCOCOCOCOCOCOCOCOCO 


—  Ol  T  '0  : 

-  ~  —  - 

T*7  '-"7  7V7  7*7  ; 


x  ©  —  oi  — 

cc 


i-Ot^-X©  —  co  t  ©  t^  © 
—  —  —  CM  CM  CM  Ol  CM  CM  CM 
CO  CO  CO  CO  CO  CO  CO  CO  CO  CO 


C  Ol  CC  L0  ©  X  ©  —  CO-* 
C-.COCOCOCOCOCOT-tf'Tf 

cocococococococococo 


©r^©©CM--t<>or^oc© 

TfTt*Tj«io*0»CiCiO»0© 
COCOCOCOCOCOCOCOCOCO 


£ 

o 

•■s> 

cc 

CO 

^> 

t-. 

• 

Ph 

© 

,  W 

in 

Tfl 

O 

"   •, 

H 

co 

CU 

0 

r<5 

■—I 

t> 

(N 

s 

iO 

•<s> 

"■ 

-1 

•^ 

I-> 

*-l 

e 

<3m 

H 

w 

q 
© 


©  ©  oi  co  -r  ©  r~  ©  c  oi 
cocototccocototototo 


CO  i0  ©  X  ©  —  01  T  '0  I- 
01  01  01  01  OI  CO  TO  TO  CO  TO 
TO   TO    TO    TO    TO   TO   TO   TO    TO    TO 


X    C  —  CO  -1"  ©  t^  ©  —  CM 

TO^l-^t'-r-^t'-f^'Tj'iCiO 
COCOCCCOCCTCTCTOTCCO 


■<*  "0  t^  00  ©  CM  TO  >0  ©    X 

iO»0»0»0©©©©©© 
COCOCOCOCOCOCOCOCOCO 


q 

OS 


i-  x  ©  —  oi  —  ■-  i-  /•  c 

rir-CMCM.CMCM.CMCM  CM  CO 

CO  CO  CO  TO  CO  TO  TO  TO  CO  CO 


—  CO  -f  ©  t~  ©  ©  Ol  CO  i.O 
TCCCTCTCTCTC't'l'r^r^ 
C0TOC0C0C0C0C0COT0TO 


©  X  ©  —  oi  -r  >~  i-  ©  © 
-r  —  —  '".  »o  'O  'O  «o  'O  © 
: :   :  :  TO  TO  TO  TO  CO  TO  TO  TO 


CMC0«O©00©-<c0-r><© 

©©©©©t^t^r^t— t^ 

COCOCOCOCOCOCOCOCOCO 


© 

X 


■o  a;  x  os  ©  cm  cc  if!  ©  oo 

CMCMOICMTOTOTOTOTOTO 

TOTOTOTOTOTOTOTOTOTO 


c.  -  ci  -r  lc  n  x  c  >-i  cc 

cc  -t-r  — t-r-ti.c  i-  it 

TO  TO  TO  TO  TO  TO  TO  TO  CO  CO 


■^©I^©©CMCOiOI^00 
lO  1C  l-O  iO  ©  ©  ©  ©  ©  © 
COTOTOCOTOTOTOTOT^CO 


OHccteMOir-cif 

t-  l^-  t^  t^  1-  t^  t^.  X  X  X 

COCOCOCCCOTOCOCOCCCC 


q 


CO  ~f  ©  t^  /  ~  —  t  -  —  c 

COTOTOTOTO-r "  —  — 

TO  CO  CO  TO  TO  CO  TO  TO  70   TO 


I-  C.  TC  01  TO  '0  C  X  ©  t-H 
—  ■—  t  7  i0  'O'O'O  '".  i~  CC 
C0T0T0T0T0TOTOT0COC0 


0 1  -<-  '0  I  ~  X  ©  -i  CO  "0  © 

©©©©©r^f-i-~r~oo 

COTOTOCOCOTOCOTOCOCC 


OC©—  CM-r^©t^©©CM 

N  N  X  X  X  X  X  X  O*  Ol 
CCCOCCCOCOCOCOCOCCCO 


q 
© 


—  oi  ~r  >o  ©  x  ©  h  ^i  -f 

TtiTf"^-^^^  —  i0   ,-i7 
COCOCCCCCOCCTOT0TOT0 


■  o  i  -  x  cc  —  to  -r  c  r^  © 
'0  'O  '0  "C  ©  *c  ©  *c  ©  © 

COTOCOTOTOTOTOTOCOCO 


CCMC0"C©X©-—  CO  -f 
l-l~l~t^l^t^l^000000 

COCOTOCOCOTOCOTOTOCO 


©t"»©©CM-*iOt^0C© 

xoooc©©©©©©© 
cccccccocococccoccTr 


© 


~  —  o  i  t  o  -r  ©  i^  ©  ©  oi 

■r 'C  <c  '"  >c  it  'C  it  c  » 

COCOTOTOTOTOTCTOTOCO 


TO  IC  "C    X   C-  —  01  -—  i-0  t" 

©  ©  ©  c  ^r.  t>  t>-  i~  t-  t-» 

CO   TO   TO    TO   TO   TO   TO    CO  CO  CC 


OC©— iCC-rH©t^©-HCM 

NCCXXXKXXffiO! 

cccccccocccccccccccc 


-ru0t^X©CMCOlO©00 

©.©©©©©©o©o 

CCCCCCCCTtt-rfiTt<-^.-f-ti 


© 


NZCidtiCNXO 
iO'0©©©©©©©t^- 
TOCOTOCOCOTOTOTOTOTO 


— ■  CC  T  ©  t^  ©  ©  Ol  CO  lO 
NNNNNNXXXOO 
COCCCOC0T0TCCOCOCCCC 


©OC©--CM-rt<lCl^©© 
X   X  X  ©  ©  ©  ©  ©  ©  © 

cocccccocccccccoco-r 


CMCO'OfflOOCr-iCO-*© 

©©©©©r-*r-r----H 
^^  ^*  ^*  *&  *3*  ^4*  *&  '^  "rj*  ^^ 


© 

CC 


10©X©©CMCC>-0©00 
©  ©  ©  ©  t—  t--  t^  t^  t^  t^. 

CCCCCOCCCCCOCCCOTOTO 


©  —  oi  -r  lo  r^  x  c  — i  cc 
!>•  x  x  /•  x  / ;  x  ©  ©  © 

TO  C*  TO  TO  TO  TO  CO  CO  CO  c* 


—  ©  r^  ©  ©  oi  co  lo  r^  oo 

©©.©©cc©©©© 

CCCOCOCC-f-'Ji-rff^f'"*' 


©■-'CC'*'©00©— iCMTfl 

-Hrtr*r«HHNN(N 
^^*p    "^J '  ^7^  ^T^  ^^  ^T  ^7*  ^r^  ^*n  ^7^ 


© 

CM 


CC-*©i^O0©— '  CC  •f  © 
TOTOCCCCCCCOCOCCCOCO 


l~-  ©  ©  01  TO  1-0  ©  X  OS  ,_ i 

X   X   ©©.©©©©©© 
COCOCOTOCOCOCOCO'v^TtH 


N-flCNXOrtMiOlO 

CCCCO-HH1H 
^fy*  ^Jl   ^^*   ^fji  ^cp   ^tj^  ^tj<  '^tj'   *cj*  ^*J1 


00©riCM-*©t^©©01 
H  h  CM  Ol  Ol  Ol  Ol  Ol  CO  CO 

^J*  ^J*  ^J*   ^T*   ^T^   ^T*   ^f   ^^  "^*  ^t^ 


r-CM'*i-O©00©rHCM-ri< 
X    S_   X   X   X   X   X   ©   ©   © 

CCCCCOCCCCCCCCCCCOCO 


i-ONXC—TCTfCSO* 

©©©.©©CO©©© 

CCCCTC-?,'rJ<'rJ'Tt<'Tj<'rt<'rfl 


©CMCC"-0©X©i-iCO-f 

T-Hr1r-*T—  WHrHClClCl 
^1^   ^^   ^^   ^J-   ^^   ^^   ^^  ^^   ^^  ^^ 


©r~©©eM-rcuot^oo© 

CMOIOICOCOCOCCCOCC-^ 

^cj^    ^CJ^    ^:T^    *7^    ^7^    ^T^    ^J^    ^T^    ^P    *^T^ 


o 
© 


©©CMCC-rf<©t^©©01 
0C©©©©©©©©© 
COCCCOCCCCCCCCCC-^Tfi 


y  '  O  -  77  X  ©  -i  CM  Tf  HO  t-- 

OCCCO-----Hrt 

TT-rfH-^'rtl-rf-TfTt'T^TfHTtl 


0C©rlCO-rr©t^©rHCM 
H  Ol  Ol  Ol  Ol  Ol  Ol  Ol  CO  CC 

^rj*   ^T1   ^7^   ^7^   ^T^   ^J^    ^3*   ^T^   ^7^   ^7^ 


TjHior^ocooiccio©oo 

CCCOCOCO-rJH-^rtirfi-^-Ttl 
^cT   ^T^    ^T^   ^7^   ^tJ*  ^t^  ^7^   ^7^  "^T-   ^^^ 


o 


CM  CM 


CC  -rfi  UC©  t^  X 


CM  CM  Ol  CM 

UO  LO  iO  lo 


©  ©         n 
Ol  CO         CO 


■*  "?  <c 

CO 


©  O        n 


1« 


Ol  CO  "**<  1-0  © 


-t<  -rfi  f  TJ-  Tf 
'      iCiOiCiC 


aso 


IC 


<—  CM  CC  -*1  >C  © 

IO  1C  1T5  IO  >C  »J  _ 

iCiO»CiO»C>C 


00©  © 

f*  »o  © 

U5«0  «0 


10163— 22— Bull.  1059- 


-13 


194  BULLETIN   1059,  U.   S.   DEPARTMENT  OF   AGRICULTURE. 


ft 

I 

o 


-3c 


SO 
re 


c 

c, 

0* 

0 

CO 

— 

- 

^w 

"0 

o 

•  - 

■- 

«o 

1- 

- 

~-o 

C 

u 

H 

S 

"<S 

I-H 

EN 

*-*■ 

1- 

-— . 

X 

-M 

— 

P 

pq 

e- 

5 

- 
> 

Q 

P* 

v. 

a 

-■O 

so 

- 

i-ST. 

■- 

E 

a, 


co 


— 
05 


co 

CO 


Ol 

CO 


q 
re 


o 

CO 


05 
CM 


Ol 


t- 
CM 


ec 
oi 


CM 


— 

"N 


CO 
CM 


01 


q 

01 


q 


OOOOOOi-H'-1'-1'-^ 


HZ ■---  rl  I-H  H  ,-H  r- I-H  H  rH  I-H  H  -  - 


_    _  -^  ^,  /-o  t*  .--  r-  or  O  i-i  «■<  t1-  t©        NO)ONWIO*»QH 

*2iN-3g§§§2«    SSSBSiSSsS    2Si322S2^S2 


SfcggSSSSSSfeS      33gg§3jSS.3S      SsSI* 


CO  -r  EC  x  r. 

■-  3  to  3  3 


S3S?g83333$    3S3SS§fcS8§    ggggggESSE 


SjgigSSSiS&SSSg    iSSS.-c.ScSSSgg    Ei_Ei_L._Z._Z;: 

I— I    —■ ti— I   — -    _ -    —    I— I  _ H  I— I  I— '  _-    —    —    —  I— I  I  


©i-ico^'Ot^^cr  —  co 

lO  Lt  iO  iC  C  »C  »C  C  C  C 


cc  to  to  to  r-  r-- 1—  i—  t-  t~ 


as  —  o  i  —  i  o  i  -  x  c  o  i  :  o 
t-ooocobococoooscs  ~- 


«OHN«iC!C«OSH 
i-CiC  CC   -  CC2CDCON 


t—  oi  o  oi  co  ■  o  3  x  ~  — 

X  X  35  35  —  3!  ~  ~  p  C 


ON050H»-f  tot-.© 
CCO©t-NNNNNt- 


o  in  w  i-c  e  '"/:  c^  —  c  i  — 
/  /  s  y  y  y  y  ■?■  3-.  35 


1 0  I  -  x  —  —  CO  —  r   •    35 

rr-  ~  ~  3  3  c  3  ~  ~  3 
—  , cicincicicici 


•f  i~  t-  S_  35  —  01  —  lO  I— 

t—  f  r-  r--  t^  y  x  x  x  x 


x  3  —  co  —  —  t  -  ~.  o  oi 

X   —   —   35  3-  3.  35  35   —    — 
_H  —  _  —  —  —  —  —  01  01 


CO  "_0  53   A_  35  —  Ol  —  53  I  - 
EN  oi  EN  M  01  01  Tl  01  01  01 


in  n  !■■:  co  t^  05  o  n  cc  o 


53  y.  35  —  0  1  —  '  o  1  -  ■/  — 
OSOOCOOOOOQQi^ 
_  , Ol  O)  O)  01  01  01  01 


—  CO  —  '3  r  -  3.  ~  ?  I  —  '  0 

ricici-i 

01  01  01  01  01  01  01  01  01  01 


&t-i0OTj<«5f-IX>Q»-HCO 

05353505051(3505000 
HHHHHHHtMtNtN 


■+  to  n  a  3  ci  c:  i'  c  / 

0000  —  —  —  —  — <— * 

01  Ol  O)  Ol  Ol  Ol  CM  0-1  CM  CM 


—  —  oi  —  i:i-/r:i:: 

—  Ol  0)  01  01  01   " 

01  01  01  01  01  01  01  :  • 


00  05  i-i  CM  CO  LO  53  CC  05  .■— ' 
05050COOOCO— i 
^H^CMCMCMCMO-IOICMCM 


CM  Tt"  i.O  I-  'S-  O  —  CO  —  '3 

— 01  01  Ol  01  01 

CM  CM  CM  0-1  CM  CM  CM  CM  0  I  0  I 


I  -  3-  —  0  I  ?'.  i  -  I  -  3.  -   — 

o  i  o  i  oi  oi  oi  oi  o  i  o  >  c  i  o  i 


:-.   —   3  I  -  3.  ~   —  -'.  —  — 

—■  — —  i  0  '  0  '  0   '  0  i  0 

01  Ol  01  01  01  01  CI  01  01  01 


tONCoCHCO-*(ONO)        C  CI  CC  'C  3   /  3  —  0-1  -t 

O  O  O  H  »-l  i 1  i-l  i-l  rH         0-1  Ol  Ol  Ol  Ol  Ol  0 1  CO  CO  CO 

CMO10<IO10<ICMCMCMCMCM         CMCMCMCMCMCMCMOI010I 


■  0  r-  y  O  —  CO  .-  I  -   /    3. 

-N*-N*^^.— .     —     —     -*     —     —     — 

oi  oi  ?i  CI  01  01  Ol  01  01  Ol 


-CI-'CI-  /-  3-Cl- 
•  0  » 0  '  0  •  0  » 0  '0  '0  53  53  53 
01  01  01  01  01  01  01  01  01  01 


-TCI-  X  3.  —  01  -r  >o  1— 

-<  ^  rn  CM  CI  N  CM  CI 

0-lCMOlCMCMCMCMCMCNC-l 


XOt-CCfONff-OCI 
O1COC0COC0COCOC0  —  — 
CMCM0-10-10-1CMO1O10101 


CO  ■  0  3  S   3.  —  0  I  —  3  I  - 

u  —  —  —  i-  i-  i-  1 0»0 

01  01  01  01  01  01  01  01  01  01 


3.  3  CI  CC  •:  3  I-  3.  3  CI 
•  '.  53  '3  3  3  3  3  3  1-1- 
01  01  01  Ol  01  01  01  01  01  01 


CM  CO  >C  CO  t~-  05  O  C-l  CO  lO 

Ol  CM  Ol  01  Ol  01  CO  CO  CO  CO 
CMO1O10IO1OICICMCMCM 


53  "/"  35  —  Ol  -t  i-O  t~  or  O  —  CO  —  3  I  -  35  3.  0 1  —  i  0 
-  -  - ■-  ~~  -*  —  -*■  —  —  —*  i  0  '  0  '  0  '  0  '  0  » ~  '  '.  "3  '3  '3  '3 
CMCMCMCMCMCMCMCMCMOl         OIOIOIOIOIOIOIOIOIOI 


i-y  or— cc  —  'Or-  (  3 
X  a;  r  -  r  -  i  -  t  -  t  -  i  -  r  -  / 
Oi  01  01  01  01  01  01  01  01  Ol 


OHW1<i.CjN«OHW 
cocooococccooo-r  —  -~ 

O-ICMOICMCMO-IOICMCMCM 


T  to  l>  C5  C  Ofl  CO  i  O  53    S 

-+-^-t  •*  icici:  iC'C  'C 

CM  CM  Ol  Ol  Ol  Ol  Ol  01  CM  CM 


35  —  01  —  '01-  /    ~   —  01 

iC  3  3  3  3  3  3  1*1'!' 
01  Ol  01  01  01  01  01  01  01  01 


-.Cl-/    30!-:"    3    /• 
,.,-t-i-r   /    r   r   f  s 

01  01  01  01  01  01  01  01  01  01 


- 

— 

- 
< 


d 

— 


/"  3.  —  Ol  CO  'O  3  CC  Oi  i— i 
coco-r-r-r-r-f-r  —  i  o 
CMCMCM0-1CMCM0-1CMO1CM 


01  -r  ■  o  i  -  f  ~  —  co  —  3 
i  0  l  O  1 0  10  >  -0  3  3  3  3  3 
CMCNCMCMC-ieMC-lOlO-lCM 


I  -  ~  ~  0  I  CO  •  0  3  /■  3  3 
3  3  I-  I-  I-  1-  I  -  I  -  '  -  f 
Ol  Ol  Ol  01  01  01  01  01  01  01 


0  1  CO  -0   3    -"    3   —  CO  —  3 

01  01   "I  " 


-—  Ol  CO  —  '0  3  !-  X  35  O         —  0-1  CO  -0  i0  3  I  -   f   3.   C         —  Ol  CO  —  "0  3  t  -  y   35  O         .—  01  CO  —  '  0  3  '  -    *     3    _ 
0  IClOlC-lOlOIO-lCMoicO         C0COC0COC0  CO  C^  "-**-*-*         -^^— ■'— .^-  —  —  — ■  —  i-         iOiO'O'O'O'O'O'O 

■O  !C  >C  i.O  i-C  iC  i-  ic         iO'OiOiOiOiO'O'O'O'O         i-Ot'O'O'O'^'O'O'O'O'O         tO'0'0'0'0'0'0'- 


RESEARCH  METHODS  IN  STUDY  OF  FOREST  ENVIRONMENT.       195 


O 


o 


24.0 

so  as  r-t  eo  th  «o"e»  os  i-H  co 

»*   '*  w  'J  '^  "vC  CO  Cfi  N  1**- 
00  CM  01  Ol  05  eO  OJ  CN  M  CO 

-  c  /  ~  -  re  t  »  /.  r 
r^  r^  r^  r  -  x   /   x  x  x  cj 

(MC<ICM01irM<M(M!M(Me<l 

—  re  >e  co  x  c  cj  re  ^e  r~ 
oaaooccccc 
eoeocMeoc^irerererere 

cs  — i  e4  ~r  o  x  co  —  re  ie 

o  — -eieiei  ei 

rererererererererere 

co 

q 

re 
eo 

rot-~os  —  co-fcor^os-* 

cococer-r-r-r-r-r-   r 
COeOCOeOClClCOCOeoeO 

e-j-f^r^c  — i^)-t-«r;x 
x    roooOoOOSOSOSOSOS 
N  C^  (N  IN  N  Ol  O)  N  C-l  ri 

os  —  re  -*■  ~e  x  co  —  re  >e 

os  o  c3  o  o  o  i ^~ 

eorerererererererere 

t~-  os  co  eo  -t-  to  x  cs  —  re 
— i  — <  eo  ei  ei  ei  ei  ei  re  re 
rererererererererere 

o 

q 

co- 

eo 

t  ^  n  a  c  n  -t  «fl  t^  o» 

1  -  r-  1  -  r-  X  X  X  X   X   X 

o  eo  t  >e  n  a  c  m  -  - 

OSOSOSOSOSOSOOOO 
eo  eo  eo  e)  ei  co  re  re  re  re 

No-wf  eza-re 

—  ~ eoei 

rererererererererere 

Cl./COM'  CC  1^  05  — 

eo  eo  eo  re  re  re  re  re  re  — 
re  re  re  re  r*  re  re  re  re  re 

c 

01 

NcO>ONCOOC)n>AN 
X    y     /    /    /   -  9  C6  O)  CO 
(NCOCNCOCOCOCOeOCOCN 

/  C  M  re  '-e  h-  CX)  O  tN  ti< 

eo  c^  re  re  re  re  re  re  re  re 

lc  n  a  c  in  ■*  ~  t^  a  — 
—  —  — .coeoeoeoeoeoro 
rererererererererere 

re  ie  co  x  co  ei  -*  m  i>  oc 

-^r^r^re  —  t-*-  —  —  -r 
^  CO  CO  CO  CO  CO  CC  CO  CO  cc 

o 

20.0 

O  — '  *^  lO  CO  X   CO  —  9SIQ 

r.  r.  *  ;  a  s  c  ~  c  o 
coeocoeocococorerere 

-  -  r  —  —  re  ic  ^  X  a  ei 

co eiei 

r-1  r^  r^1  re  re  re  re  re  re  re 

re  >e  r~  x  ~  ei  —  ic  t^  os 
e*^  e^i  ei  ei  r*  re  re  re  re  re 
z"  r^  r^  r*  re  re  re  re  re  re 

-^  re  ~r  -o  x  co  ei  re  >~  i^ 

— *■  ' —  —  » e  '  e  '  e  '  e  i  e 

CO  CO  CO  CO  CO  CO  CO  CO  CO  cc 

o 

L9.0 

x  o.  —  ^-^sJ^iCS^co 

-v  EO  X  ~  —  r — f  -C  x  o 

—  —  cocococoeor^ 

re  re  re  re  re  re  re  re  re  re 

—  re  >e  -»c  x  co  ei  re  >e  t- 
rererererererererere 

Os--iCM^cOOsC0  —  me 
-f  »o  'C  >e  ie  ie  c  c  c  co 
rerere  re  re  re  re  re  re  co 

d 

q 

X 

c  t-  a  -  N  ■*  to  N  O!  -1 

CO    CO    * I — 1  — 1  r— *  CN 

cocoooeocoeoeoeoeoco 

e  i  —  --=  r~  C5  —  ei  —  :r  x 
ei  ei  ei  ei  ei  re  re  re  re  re 
rererererererererere 

os  —  re  —  o  x  co  —  re  >e 
rererererererererere 

NaONtc  x  cr.  —  re 

'C  'C   CO  O  CO  CD  CO  CD  N  N 

rererererererererere 

o 

17.0 

-r  ■-  i-  OS  —  ei  ■*  i.-  r-  OS 
N  cNCNtMCN  Ol 

re  ^t  re  x  r?  re  re  re  re  re 

co  e-i  -f  "3  i^  ss  c  e-i  —  — 

rererererere  —  "*- 

rererererererererere 

t^  os  —  eo  —  co  x  os  —  re 
—  —  '0  '  e  i  e  '  e  •  e  » e  cc  cc 
rererererererererere 

ie  i-  z  o  n  ■*  cc  n  a  -j 
co  co  cc  t  -  i~  r~  r^  r-  r~  x 

re  re  re  re  re  re  re  re  re  re 

o 

o 

CD 

ei  so»Ol-»  x  ro  ei  re  ifl  i  - 
e  i  e  i  e  i  e  i  ci  re  re  re  re  re 
re  fe  fc  fe  fe  re  re  re  re  re 

r**  re  re  re  re  re  re  re  re  re 

>e  r~  —  co  ei  —  vc  i-  ~  — 
iOU3iOcOcO<OCOcO<Ot~- 

r^  r*  r^1  re  7^  re  re  re  re  re 

re  '0  co  x  cro  ei  -r  >e  r^-  os 

1^  t~  t^  t-  X    X    X   X   X    X 

rererererererererere 

o 

q 
i-' 

C  —  CO  *0  CO  OC   C  —  re  •  * 
M  re  re  re  re  re  —"■  -**  ~**  ■** 
*^*  r-"  r^~  re  re  re  re  re  re  re 

■-  X  ~  —  re  •'  -s:  f  ~  ei 
^**  "*■  i*  «e  i*  <e  *e  »e  ^o  ^e 
r**  r^1  re  re  r^  re  re  re  re  re 

re  ie  n  x  c  n  -f  i.e  n  a 

CO  O'OONNNNN  t^ 

e**"  *^  ^^  r^  r"  re  re  re  re  re 

—  re  t  co  z  o  ci  re  ie  n 

X   X    X   X   X  ~  OS  OS  OS  as 

r^"  r^  re  re  re  re  re  re  re  re 

o 

q 

r  r:  — 4  re  -r  co  .r  cr.  -—  re 

re  re  -*■-?•  -^  —  —  —  » ~  ■  e 
r^~  re  re  re  re  re  re  re  re  re 

c5 

«C9XO>HCO«C    X    c 

'i  it  ie  '*  'O  'O  -  'O  -  i- 
r^  re  re  re  re  re  re  re  re  re 

i-tCOiOCOOOQOmiQN 
t-[-i-|^l-X    /•    /•    /"    /• 
re  re  re  re  re  re  re  re  re  re 

os-iN'fcozo-re'e 

zaaaaaooco 
rererererere"i*TfTr't 

q 

re 

■;i-r.-Mf -o  r~  —  — 

t^^h^<ICkOU3lOlO>A   CC 

rererererererererere 

:i-oi----i~';   < 
tO(OCC  r  i-r-i-i-i- 
rererererererererere 

r.  —  re  —  ~  s  —  —  re  >~ 
\~  s  /  /  X)  oo  os  os  os  o: 
rererererererererere 

t^oscoei  —  co  x  cs— 'co 

ososeococo:co~c:  —  — 
re  re  ^"  *^"  —  ~~       —  ~ *"  ~ *" 

o 

q 
-1 

i  e  •  -0  » o  » e  co  co  co  co  co  co 
rererererererererere 

o  ei  -r  ie,  r^  c.  eo  ei  —  -,; 
i-i-r-i-i-i-x   r  r  x 

i  -  ~  —  ~)  —  —  x  a  —  re 
x  x  os  os  er.  cr.  os  os  o  c 

-^  -^  ^~  -^  -^.  -^.  -^  -^  —  — 

m  i_-  x  —  ei  —  co  i  -  os  — 

—  —  ^f  —  -f-f  —  —  -r^t< 

d 

11.0 

oireier-xrocire'Cr- 
•  -    -  •.-  cc  cc  i  -  i  -  t  -  t  -  t  - 
r^  re  re  re  re  re  re  re  re  re 

r  ceireiei-x  rrei  — 
1  -   f    S    f    s    /    s    -.  ~   ~ 

— ^  r^*  re  re  ?~  re  re  re  re  re 

'J;  Lr  Z-  S  i.1  ~  =  —  S  — 

rerefe^"*~"'*""~*"— ■ 

re  >o  co  /  cn-fieNa 
ei  ei  ei  ei  ei  ei 

*■*■  "■-?"   ■"*■  "T"  "~"  '^T"  "*"f"  ^"t"  ,*^"  ""T 

o 

q 

d 

— 

co  —  re  >~  cc  /  —  —  re  >~ 
r^t^f-i-i-t-  r   x   /   x 
re  re  re  re  re  re  re  re  re  re 

•o  /  :  -  r":  o  /  r  m 
f  r  ~  —  ~  r-  os  os  o  o 

-rf-    -^   -^   -^    -^*   -^   ~*~    ■^-«  ^y.   *^. 

r_-  ie  r_;  x  CO  e i  -r  W  t~  OS 

—    —    — -    — —    — —    —    — —    — «-    — -    — 

— <eo-fOcco<Nce>e  r- 
ei  ei  ei  e i  ei  r*  r^"  re  re  re 

—    —    —    -*-    — *-    — -    -^    »J4   ^rj*   Tf 

d 

q 

OS 

X  os  —  re  — •  so  X  3j  —  r^ 

i-  t-  x  x    /  x  x   r  —.  o. 
***  ~^>  -^  «^»  -^  -^  *o  -**  -^  -^ 

—  ;r  f  — .  —  re  —  o  /•  ro 

aaaaooccc^ 

—*  re  >e  -c  x  co  ei  re  'e  t- 
eieieieiei 

_—        _—        _»        «        _»        •*.       V^-       ■»*.       T^. 

a>  i— i  <m  •**  co  x  o  —  r^  >  e 

eirerererereT-f  —  — 

W     v*.     -—     — +«    -»*.    ^-     -Hm     »*«     *— .     -^i 

o 

q 
x 

cor^cjs— •co-t'cor^os-^ 
/•  /   cososososososo 

cerererererererere-r 

ei  -r  -.0  r  -  ~  —  ei  -r  ■■£  X 

COCOO-' IrHH 

""  "~ "  ~" "  ~* *  "~  ~ *"  "*"  ™  ^f  ~T" 

os  --  re  -f  'JO  x  co  —  re  >e 
—  ei  ei  ei  eo  ei  re  re  re  re 

re  re  —  -r  —  -r  -t*  *t*  *e  »e 
^*  ^^  *t*  ^t*  "^i4  ^*  ^*  ^&  ^*  *^t^ 

<-> 

q 

—  >-  r^-  os  o  ei  -r  *e  r-  05 
OS  OS  OS  OS  o  o  o  o  o  o 

rcrcrcrr-t-ft-t-r- 

co  ei  -r  ■-  i-  z-.  —  ei  —  ■- 
i —  ei  ei  eo  eo 

* —     —     —     — —     — |l         -    — J1     —    ~^»    Tf* 

r  -  — .  —  e  i  —  vro  x  cr.  —  re 

^  ^  ^  -^  -v   *v  -^  ^  -*■  •*■ 

r-  —  —  -^r^-r-f^t"*1 

i:  n  /  c:  e-i  -^  o  i^  cr.  — 

—  —  -fir"  » e  «e  <e  'e»e  co 

O 

q 

CD 

N  eo  «o  h-  x  co  ei  re  'O  ^- 
co  co  co  co  co  --  —  

^^  ^*  ^*  *&  ^*  "*4*  ^*  *^   ^*  ^t* 

x  —  e  i  r'  i  e  t  -  s  co  cm  -t1 
—  oieieieieie^rerere 

—      —    ——     .  1 1    ^+<    ^^"    —     -  * '    ^**     ■  J ' 

ie  i-  a  c  ei-r -o  t-  a  — 

~^-^'^— ?•-*-  —  —  —  -*-ie 

—  —  —  —  — —  — 

'  e  » e  >  e  ■  e  co  co  co  CO1  co  co 

~f  "^"  *^   ~*"  "f  """f  "T*  "T"   ~+*  ~T* 

o 

q 

C  HCQUSCXQ  —  re  i" 

—  —  ei  ei  e>  ei 

— —    — — -    —    —    «-»-    ——    -»—    —    —    — — 

C  30OHCQ  'C  CD  X  —  CN 

c^ic^cecerererere  —  — ■ 

^*  ^^  ^*  ^i*  ~*&  ,*4*  "*v  *^"  ~^"  *^* 

r.ei-y  co  e  i  —  1:1-0 
~f  -r  -t-  —  >e  >o  >e  i.e  >-e  ua 

"™^"   ~?"   ^^*   "™f   '*T   ^t*   ^3*   ^t*   ^"T*   "T^ 

^-  'v*  — "  *x  x  o  n  :*  <*  i- 
co  \f  co  co  co  t  -  t  -  i  -  r^  r^ 

■rf^  ^H  ^H  ^^  ~f  "^*  """**  ^*  'rJP  ^T* 

o 

q 

x :  o.  —  re  —  cc  x  —  —  re 
—  —  ei  ei  ei  ei  ei  ei  re  re 

■^  ^t*  *^  "^  ■*tH  ^^  *&  ~&  *&  ^t 

-  ■»  /  r.  -  ^  t  c/  c 
rererere" —  —  —  —  —*.,* 
^V  "^v  "^t"  *"t*  *^  'T^1  *t*  *i*  ^f  ^t1 

—  ?e  ic  *  x  c:  ei  re  »e  t^ 

»e  '.e  ie  »e  >.e  "^  ~c  '.z  —  — 

■^i™  ~—*~  ~~*"         *"•"  ■"*■  ~^"  "—"  "—_  """" 

0-iN'fCZO-«iO 
CD  N  N  N  r-  t^  X    X    X    X 

^t*  "^t*  ^it^  "^f  "*&  'T*i  "^"^  "^f  ^^t*  "^ 

o 

q 
ro 

SNOB-Jfl'I'ONCS-H 

cvlCMevirerererereret" 

1  T '       ■ '   ^rj^  ^^  ^T^  ^t^  ^T^  ^^  ^^  ^7^ 

C^  Tt  CC  N  Cl  ^  M  t  •-  X 

—  —  -r  —  —  ■  e  i  e  >  e  » e  » e 

^^  "rf"  ^7^  ^f"  ■—^"  *™f"  "1"  ™"T"  "Tj*  't 

33  —  -"  —  %Z  /  c  -  ^  i* 
i.e  ,r  co  —  co  *  t-  r-  r-  t— 

^T"  ~T"  "™^*  "™t*  ^^  ^j"  ^r^  ^T  *cf  ',^^J, 

i-aoei-oz  a-re 

(~  I^  X    X    X    X    X    X   OS  OS 

Tf^   Tf   ^f   "*^   ■*+"   *~t*   ""**    "^ T"    'riH    "^ 

o 

q 

CO* 

— *  <e  r-  cr.  —  ei  -r  ^e  r^  c^ 

ceTerece-^-r-r-*-"^""*" 

^f   ^J*  ^J^   ^^*   tJ*  ^t}*  ^t   ^J*  TJ*  TJ* 

C  ^  -t  »C  N  a  C  ^1  -t  '>£ 
i  e  i"  i  e  i  e  » e  « e  w  ^c  —  » 

^^    ^^    ^?*   ^^    ""^    "^?*    """I*    ^"J*    ^T1    ^T* 

t^  o>  —  ei  —  o  x  —  —  re 
o  o  t~  r~  r-  r—  t^  r-  i  ~  x 

—  —  —  —  -r-r  —  -r  —  — 

i.e  n  z  c  ei  •*  o  t-  a  - 

X   X   X  OS  O-  O.  O.  O-  O-  CO 

^ti-*-rf-t-r,'*-^*'*-r-i-e 

£3 

q 

NM>ON00OO)CQ>Ot> 

— r  — »■  -rf  *+<  -rf  iC;  »~e  *e  '  e  >  e 

^^*    ^^4    ^^^    ^^    ^^    ^7*    ^T^    ^T^    ^7^    ^^ 

x  co  ei  re  >-e  t-  x  CO  eo  »ti 
i~  co  to  --C  O  XICON  t^  t^ 

^*    ^^    Tf^    ^t"    ""T"    ^^    "^^    *^    ^*    ^t* 

>ei-a  coeo-*^:t^05— < 
i ,  i  -  i  -  x  x  x  x  x  x  cr. 

"^*   ■"^"    T^H   ^"   ^f*  ■*$*   tJ-   ^*   ^*   "*i* 

re  i.e  co  x  co  ei  -i*  ie  r~-  os 
aaaaoocooo 
^f  rt*  tji  io  i/ej  io  i.e  »o  »o  to 

o 

p 
d 

OHW»O«D00QH«ifl 

i  e  i"  i  e  '  e  •  e  *  e  "^  xc  cc  ^c 

— *.    -—    —     —     ^     —    ^i   ^fi   ^*   ^J4 

uo  x  co  —  re  <~  —  x  c  eo 
CiM-  r^  r^  lr-  t-  X  x 

~" T"   "^"   "^   ~?"   ^i-   '^"J'   ""t*   "^t"   '^"   ^^" 

:e.et^/CN-f  >-e  r^  cr. 
/  /  z  /  a  a  a  a  a  a 

~~*  - **  "^  ^t^  *^  ^i  ~^*  —  —  *^* 

-  re  -t  c  z  c  n  re  ie  t- 

ccccc — 

lo  lo  »e  ie  ie  'C  »o  i.e  »e  »e 

O 

3 

CD 

: 

— j  ei  cej  •#  >c  p  t-|  x  q  q 

—  "-C  '-^  "-O  »  *  —  '-~  —  t  -- 

io  >o  *o  *^  *-e  >o  ^  o  »o  io 

th  ?i  re  -t  o  -o  n  "X  d  d 

t^  t-^  t^  r^  r-  r-  r~  t-  t-  x 
•o  i-e  >e  'e  >t  <t  ui  ui  <~  ic 

-HiNre-t  i-e  p  t-|  x  q  q 

X  X  X  X  X  x  x  x'  X  OS 

•  e  ie  i.e  ie  ie  >e  ie  >e  »e  io 

-j  eo  re  -r  ■  e  p  t  -  x  q  q 

oc^c^o^osca;osososco 
lc  lc  ie  i.e  'C  ie  'e  <e  >e  co 

^  - 
•8    £ 


^    5 

-7     PQ 

8  •      K 


196 


BULLETIN  1059,  U.   S.   DEPARTMENT  OF   AGRICULTURE. 


CN 


CO 

CN 


cocococococoeocoeoco 


tc  t-  o  —  cc  lo  i»  c-  •—  cc 
Tf  -rf  -r  lc  i-t  it  i-t  it  cD  53 

cocccocccocccccccocc 


u~  i  —  3.  ' —  CO   » O  t^»  05  ^  CO 

CO  ffl©NNt-NN«    /" 
cocococococoeocoeoco 


■  t  i  -  3.  —  co  — ■  -3  x  ©  oi 
x  r.  /■  r.  3.  3.  o.  3.  c  c 
:■:  r:  ^  ^  ?r  :'.  r.  r: 


iO  t-  35  O  M  —  53  ^  ©  CN 

cococccocceccccoccco 


Tfutr— Os^-cOLtr^-cci-t 
it  it  113  l-  ©  to  cC  ©  CO  t— 
cocccococceococecocc 


co  lc  r-  35  —  co  lc  r-  05  — 
i„  t-  r—  t~  y  X  X  /■  X  3- 
cococccccococococcco 


-o  1 1  I  -  3-  —  "O  •  0  t  -  575  — 
3.  3.  3-  3.  03  O  ©  3  ©  — 
co  co  co  co  -r  -r 


p 
IM 


T3 
O 

- 

•i-H 

— 

PI 

o 
O 


to 


■HO 

►O 

r-o 

^5 
to 

o 

•  — 
co 

s° 

*~ 

■£> 

~3 


Si 

:5- 

to 

a 

O 

e 


CO 

I 

co* 

s 

CO 
CO 

CO 

a, 

O 

a. 


o 

CN 


O 


O 
00 


p 


p 


p 

id 


O 


p 
CO 


— 

© 

O 
ft 


o 

© 

ca 
iJ 

pq 

ft 

w 


o 
oi 


p 
© 


o 


p 
00 


O 


p 

co 


p 


p 


p 

CO 


COiOt^0C©5NTT<©  x  © 

Tf    f    " "   '"    ''■    '~    L~    '"    — 

COCOCOCOCOCOCOCOCOCO 


cn  co  »o  f-  o  i— i  co  io  t"»  O) 

cccccccccocccococccc 


—  -o  it  r~  35  —  cc  lc  r~  o- 
X  y  X  /  /.  5  C  5".  35  35 
CO  CO  CO  CO  CO  CO  CO  CO  CO  cc 


—  CO  ■-  t-  35  —  I 

C  ©  C  C  ©  —  • 

—  tf   -f   T   f   *"C   ' 


:  •.-  n  c 

—  ~-  «--  •— ■ 

-  _  _  _ 


HMiOffiOCODtfC* 
IC  it  it  '  t  it  53  —  ©  cc  — 


©  •— i  CC  i~  r—  35  —  CO  >0  i-- 

r^  r^  t^.  r^  t--  r~  y  y  x  x 


35  —  co  it  t^-  C5  ■— i  co  it  r» 

xosscssiooco 

cocoeocococ-v-r-T-r-r 


35  —  CO  l-t  i~  05  —  CO   '  t   1  - 
S 7-1710101 

—       _—        —       «■       ■_*       -w~       -—       •-■       ^      ^ 


COCOCOCOCOCOCOCOCOCO 


0C  C5  —  CO  lO  t^  32  —  CO  l-O 
t^  t^  S.  30  00  oc  >:  ~  35  C5 
cocococococococococo 


h-  3>  —  CO  i0  t-~  32  —  CO  '  0 
3)0)00000 

C*C  ^*^  ^"T*  "7*  ^^*  ,,!J,  *^  ^^  ^T*  ^^ 


t^S5  —  CO  1 0  I ~  3".  —  ''.''. 
—  — -rjoioioioicococo 


C3  w  tt.  t^  t~-  I"-  t—  00  3C  00 
COCOCOCOCOCOCOCOCOCO 


CO  It.  C3S  i — '  CO  L0  t^»  35  —  CO 

y  y  /  ~.  ~.  ~  C-  ~  ~  ~ 

cocococococococo-r-r 


'O  t~-  35  —  CO  L.0  t<-  3.  —  CO 
C  O  C 01  01 

^*   Tf*    ^^    "^    tj"    ^»    t^<    tf<    ^*   ^H 


i.o  i^  pa  — 

oi  oi  oi  co 

^,  ^*  ^*  t^< 


CO  ■  O  I  -  35  —  CO 

-~.  —  co  co  tj<  tf 
—  :-. 


iOb-C3>OCMTj<COOOOC<( 

NNt-xx/;  y  x  35  35 

COCOCOCOCOCOCOCOCOCO 


-t-  iO  r^  3".  ■ —  CO  LO  t^-  35  — 

C7CCCCCC"tTT"^*""" 


-"-  io  n  r.  —  cc  C  t-  c;.  — 

—  —  ^-  —  0 1  0 1  ~  1  0 1  0 1  CO 

—  —  -r  —  ■— •  —  — 


cc  VM-  O  -  cc  l:  t»  O)  t 
co  co  co  co  —  —  —  ■—  -*•  i.t 

^T   "^   ^*  ^*  ^*   ^^  tf  ^*  ^*  ^* 


wmttoocNTfCceo 

00000000353235O535C: 

cococococococorccotr 


CM  CO  '0  l>-  3.  — 

c  co  o  =  =;  — 

^*   Tf   Tf   T^    "*"    ""*" 


re  '"  I  -  r*.      — 


—   —   —   — 


J  t^>  "  —  :t  •*  t-  Ci 

*)  ?i  71  ?i  ?i  r"   : 

~""    ■"""    ~~~    —"    T^    ■"*■    '"— "    "™"    *™ "    "™" 


—  —  —  —  —  I-  '  ~  I  ~  » "  ■" 

^mmm     ^**     ^^    ^*"    ^* "     ^^*"    ""^    *"^    ^"^    *^ 


rfMWtccccfi'O'-  y 

05053535350COOC 

coccccctcc^'t'^'t^ 


O  ^^  CC  i-O  t--  35  —  CO  '0  l~ 
^  —  . —  01  01  01  01 

— .  —  —  — —  —  -r  -^f 


05  —  co  i~  r-  35  —  co  ■  o  i  - 
^■^"cccccc  —  —  —  — 
: . . -  — 


35  —  CO  "0  t^  35  —  CO  •  0  r  - 
—  tOiOtQiOtOCOeOCOcC 

— —      v^      »•-     •-**     »*     ~^     ^~     ^^     ^**     *■* 


05  ^j  co  -f  53  oo  ~  oi  —  vr 

3500000 

r*C'  ^f  ""T"  "*T  "" 7*  """*"  ™~ "  """"  "" "-"    ""*" 


OC  35  i-t  CC  >-0  t^  35  —  CO  'O 
01  O)  01  OI  ?1  CO  CO  CO 

-  — 


I  -  35  —  CO  •  0  I  -  3.  —  CO  ' " 

co  co  —  —  —  -  —  ■"  '0  io 


1-3-  —  CO'01-3.  —  CO'O 

•  o  ■  o  o  o  3  o  vr  i  -  i~  r^ 

~~~    ^*    ^*"    ^^    *"*"    ^*"    ^*    ^T*    ^■^    ^^ 


I>  O!  — I  OI 

O  O  ^t  ' — ' 

Tt^   tj-    -^"    — ~ 


—  53  y  O  0 1  -r 
01  OI  01 

^Jl   ^rjt  ^rj*   ^cj"    ^Vj-   ^T^ 


^:  I—  *  —  re  »-~  t—  C-  ■ —  r* 
N  C^  ^^  CO  CO  CO  CO  C3  —  — 

tJI  ~7*  *T  ~f  "^  ^  "^"   "*"  "^*"  ~* 


iO  t>»  OS  —  :*>*!-  C  —  re 

—  — -■— ,*,'",e>e,e»"» 

_    _    — .    —    — .   —   — .    —    —    — 


<:n  cs  —  re  »~  r-  c.  —  re 

■-  ■-  -~Z  t  -  t  -  i  -  r  -  t  -  r    / 

TT  —-*■   —   —   —   —   —  — 


»ONClC 

1-H  1-1  ~H  O) 

Tf  ^rf<  ^1  Tjl 


c^  !M  cm  c^  re  re 

■■•cyt  ^rj*  ^J^  ^rj^  ^cji  Tr*l 


•t'CI-O- 

co  co  co  co  — v 

—7"  "^  ""*■   ~ —  ~~ 


CO  >0  1-  35  — 
—  —  —  —  iO 

~  — .      w    —  — 


CO  '0  I-  3-  — 

» O  t  O  '  O  '  ~  o 

_  _  _ 


--  ■-.  i  -  r.  — 

3   ■-    ■£    3  1- 


CO  '01-3.  —  CO»"l-35  — 
I  -  I  -  I  -  i  -  /    /   y    f   y  ~ 

—  —  — rTf 


co  >-o  i  -  y 

(N  <N  01  01 

tfl  tf  -*  — 


om-3  y  o 
co  co  co  co  c~  ""c* 

-*.  -^-  ^-  M-  -^-  -* 


01  CO  '"  I-  3.  —  CC  it  I  -  3- 

—  —  — it't'" 

__ __ 


—   ■  ~  ■  ~  I  -  3.  —  CO   '  t  I  -  3. 

CO  CO  CO  cc  53  i  -  r  -  i  -  i  -  i  - 




—  CO'tl-3.  —  r-'tl-3- 

f   r  r   /   /   z 
— 


i-H  CO  'O  53  y  O  01  —  53    X 

cocococccctt-r-r  — 

^*  ^^  ^T^  ^t*  t^*  Tt*  '^  ^T"   ~ *  """ 


C--CC't  1-3.  —  CO  '.O  t— 
■  t  it  '0  't  't  't   50  '3   '3   "3 

•*   **"    "" T"    "^"   "" "**   ~^    ~~"    '"^    "*"   ""^ 


3.  —  re  it  i  -  3.  —  cc 

53i-i-i-i-i-y  /■  /■  ' 

—    —»   — ~   —    —    —    —    —    — . 


3  —  -:•"!-  3.  —  - 1  ■  t  r  - 
:::: 

—  —  —  —  —  —  it  1*3  it  it 


0)HCC-t3X3Cl-3 
co  ■"#  t(<  -*1  ~r  ~r  i-o  ■  t  i-0  lo 

^71    ^cjH    ^cjl    ^cj^    ^cJH    ^^    **Cj1    ^57^    ^^    ^cj^ 


OC'  Ci  i— '  re  »*  i  -  C-  —  r*  •  e 
» e  1  e  *c  ~  ~~  —  —  t  -  t  ~  t  - 


1  -  3.  —  ct  1  o  r~  3.  — 
t^f-ooocooooacc&OJO! 


I  -  3.  —  •-  it  I-  3.  — 

C3333  —  — 1  1— 1 
—  —  .  t  '  t     "  ■  ".  1  .  •  t  iO  10 


t^  co  —  oi  ~  53  y  o  o-i  t* 

—    —  '  t    1  t    '  t   1 1    '  t    53    53    53 
^  "^i  ^  ^f  tt^  ^  t^*  ^i  ^  ^}< 


53  t^  3.  —  Ct  ■  t  I  -  3.  —  CO 

3'33NM-l>l-  y    y 

t*irji -  — 


lo  r-  3.  —  ct  ■  t  1  -  3.  —  co 
r  s   y  ~  ~  ~  ~  ~  —  — 

—   —    —   —    T   —  -   -  iC   1" 


itr-3.  —  :  t  •  t  1  -  3-  —  ■ " 

~   ~    ~ 01  01 

it  it  it  it  it  1".  •".  it  1O1O 


io  n  c:  o  c-i  1 3  y  o  01 

UO  UO  ut  53  53  53  53  53  1  -  I  - 

—     ^^<     ^J^     ^J<      — ».     .•     — —     V*.      —      — ~ 


—   itI-3.  —   CO»tI-35  — 

1-  t-  t-  i~  y  y  y  y  /  3. 




--  it  I-  3-  —  "  1*    '  -   3    — 

~  -  -  -  ~   z  -   -  ~  — 

—  —  —  —  1 1  I  \ 


-  3.  —  Ct  it  I  -   3.  — 
01010I010ICO 

■  "  it  '  t  ■  "  it  it  it  it 


«WNCOOIN-*COOO 

^rj^  ^rji  ^trji  ^fl  ^Ji  ^Ji  Tf^  ^^  "*rj^  "^ 


CNCOL0t^35  —  COiOt-35 

y  y  f  y  y  3.  3-  3.  3.  3. 

— C"  - ~  TJ1  *T  — r  -T  -c  "*"  "*  T 


—  :-itt-3.  —  :C'"l-3- 

—  ~  ~  ~ 

It    Lt    it    It    it    it    It    '  " 


—  cici-r.  —  co  1 1  t—  C5 
citioioiticococococo 

i  t  it  it  ■  t  it  it  it  it 


r-1  CO  IO  50  00  C  Ol  -f  53    X 
NI-NNNMXWKM 

^rji  ^cj<  ^Jl  Trp  ^51  ^?ji  Tj*  ^rj^  ^J*  "•rji 


O  —  CO  1-0  t—  35  —  CO  it  >~- 

05  C5  35  35  3-  3-  O  ~  O  O 
-c-r- it  it  it  it 


35  —   ~^i~!-~5   —  COitl- 

o  — ointiri 

Lt    Lt    »-t    't    •  t     ■  t     •  " 


3-  —  :  t  '  t  t  -  3.  —  c  o  ■  t  t  - 
01  co —  — 


0>HCC^tCXOCT-3 
t^00O000X0C'35O5C5C5 

^Ji   ^J1  TJi  ^ijl   ^cj^   Tp   *cj1   ^J^   ■^jji   -^< 


00  C5  —  CO  it  I  -  3-  —  ct  ■  t 

0)0)00000 

"*?<  -}■   Lt    Lt    Lt    It    It    I  t    l-t    It 


t--  C5  —  CO  1 1  t  -  3-  —  CO  >t 

01  01  01  01  01  co  co  co 

Lt    Lt    it    it     '  t    •".    It     it    it    '  t 


1-3.  —  "C'tl-3.  —  COit 

—  .  t  >C  i-O 

itltltitititltl- 


NO)-ilNT)iffi«Oclil 
OOOOG5C505053500© 

rti     .  —  -r  -t-  —  ~f  it  it  it 


C3  t^  C55  —  CO  it  1-  3.  —  "t 

000 0101 

it  it  it  it  it  '  t  '  0  it   1 1  •  t 


It  I-  35  —  CO  it  I~  35  —  CO 
01  01  01  CO  CO  IO  '"  ct  **"  — 

't  it  it  ifi  it  it  i?i"i  it  it 


itt-3-  —  c-itl-3.  —  CO 

— •--—  —  1"  it  O  O  '"  '3   '3 

I  t    1  t    1  t    ■  "  i-t    Lt 


lOSOiOM-t5CKCN 

oaoioocooHH 

*S>   tf    -f   Lt    Lt    Lt    Lt   Lt    Lt    Lt 


-*■  lo  1^  35  —  co  it  r-  3.  — 

^H  ^t  — h  —  01  01  01  01  01  CO 

Lt    Lt    Lt    Lt    Lt    Lt    '  t    it    it    '  t 


co  >t  t~  35  —  y  1 1  1  -  3.  — 

co  co  co  co  —  — "  —  '  t 

it    '  t    It    '  t     it    '  ~    '  t     it    It    Lt 


c^  »t  r-  3.  —  co  '0  1  -  3.  — 

1  o  1 1  1 1  1 1  53  53  53  53  '3  I  - 

It  It  It  't  It  It  It  It  It  it 


OS 

- 
m 


o 


co  it  r-  -y  c3  oi  —  53  y  o 

OOOO— .  —  —  ,-irtCN 

IO  »0   Lt   Lt    Lt    Lt    It    L~    I*    1- 


0 1  CO  1 1  I  -  3.  —  CO  1 1  l-  35 
OI  CM  01  Ol  Ol  CO  CO  CO  CO  CO 

i-O  it  Lt  i-t  it  it  it  iO-   i"C  'O 


—  cc  i-t  r ~  35  —  co  1 1  1  -  3. 

— .-  ,-  ,-  ,-,- 

ltlt'tlt'tltl~'t't't 


—  coitr-  3.  —    -    ti~Ct> 
i     C     -     -"    3  t  -  t  -  t  -  t  -  I  - 

'"  It  it  It  It  It  it  It  It  it 


o 

0 


CD 


r-MlCCXCNt-3    X 
--!  -t 01  Ol  Ol  Ol  Ol 

IO  IO   It    Lt    I-t    It    It    Lt    It    Lt 


O  i—  CO  it  t^  3.  —  co  it  I- 

cocccccococo  —  —  —  — 
l-t  I-t  It  It  It  it  It  It  It  It 


35  —  CO  •  t  I  -  3.  —  CO  1 1  1  - 
—  '  t  1 1  1 1  1 1  1 1  53  53  3  3 
It  It  it  It  It  It  It  It  It  It 


35  —  CO  1 1  I  -  3.  —  :"  ■  t  t - 
531-I-I-I-1-''  '  r  X 
l~ltltltltltltl"' 


C5-  W-feorjCClrfO 
--CNO<1CMC<1CMCOCCCCCC 
IO  HO  It   it  Lt  it  l-t   I-  l~  1~ 


y   3.  —  co  •  O  1  -  3;  —  CO  iO 

Moot  —  —  -r  —  1 1  1 1  1 1 

It    l-t    It    Lt    l-t    It    It    l-t    It    Lt 


r~05  —  ct  it  1-  o.  —  co  'O 

1 1  1 1  "3  53    3  53  53  t  -  r—  1*- 

lt     It     It      't      It      It      I*      it      It      It 


r  -  9  —  -"  ' "  1  -  3.  —  cc  kC 
t-  1-  r    f    r    f    /  3  3-  3 

1:1:'-  >'  t  '-it''    ■ 


"*.  Ci.  —.  "t  '5  — !  H  °0  Ci"  P;       r-i  O-i  CO  -r  1 1'  td  1  -  »  3-  3 


0  p  p  O O  COOOH 
CO  CO  CO  co  CO  CO  co  CO  CO  CO 


.    (i—l^,—  — ^  —  ^  —  ^-rv) 
53  CO  53   53   53  53   53   53    '3    53 


— '  oi  co  — •  1 1 53  t*  y  oi  0 

•  1  - 1  - 1  - 1  - 1  - i  oi  co 

53  "3  "3  53  CC  53 


CO  cc  co'  CO  CO  CO  cc  c- 


RESEARCH  METHODS  IX  STUDY  OF  FOREST  ENVIRONMENT.      197 


"O 

<o 

o 

6-i 


CO 

P 

pq 

E-i 


24.0 

-r  a;  ac  —  re  io  r—  as  — i  ee> 

^5^  ^=j*  ^^  ^r  ^7^  ^t^  ^^  j^t*  **?*  ^^ 

c 

q 

CO 

CC  IO  O  ©  — '  re  >e  t»  as  — 

rH PJ  <M<N  n  ei  en  re 

r. 

© 

rH  cc  io  oo  as  t-i  eo  »o  t»  as 
tNe*eMcMNeoeocoeocc 

— ^"  -^  - r*  *^"  —  — —  —  -^ *  — ^*  ^* 

o 

21.0 

OS  —  PC  *-^  r *»  c^  —  re  ■  0  r*- 

^^  ^t*  ""^  ^?*  *"^  "™t*  "^*  "^  ^*  ^^ 

o 

o 
© 

t^-C-—  -*  i*  N  C  -  CC  ' t 

-~   --—  —  —  _-*.  ,~  ,~  ,-• 

"*T^  ^T*  ^T*  "7"  '"?*  "7*  ~J"  """?"  ^7"  *"t* 

o 

o 

■  e  r  -  —  r i  —  —  x  CMt 

—  —  —  >~  '~.  >~  '  e  cO  cC  err; 

^*"    ^*    ^*"    **     ^* *     ™*    ^^    ^*"    ™—    ""*" 

c 

q 

00 

?-  i-  i-  c  ri  —   -  r  cr  rj 

."'"»"   -"   'JT   CC    ' ~   -_~  J  -  h- 

—    — •-    —   —    —    —    —    -—    —    — 

c 

q 

—  M  »o  0C   OtN  —   -T   X  © 

£  EC  Eg  C  t  -  t  -  r  -  i  ~  t  -  A 

o 

q 

to 

i— ( 

©  —  re  -z  X  —  ri  —  ec  y 
-O  i  -  r  -  r  -  t  -  y   r    y    *    X 

H^  ~*<  T*  "~f"  ""?"  T  T  ~~ f  ~~ T1  *4* 

d 

q 

t~-  c. etc  y  rr  ei  -*  e 

i -  i  .  y   /•   r   jo  OS  Os  o»  as 

© 

q 

-H 

>e  r-  os  ei  —  to  y  c;  ei  -n 
/■  y  y  —  r.  a-  a.  ~  ~  ~ 

— ■  ■—  —  —  —  _■_,-,-,-_ 

o 

q 
re 

re  >e;  r^  ~  e  i  —  --  xce^ 

9  0150CCCC-  — 

Tt<  -i>  -*■  ■  e  ■"  '~  '~  v.  >~  iG 

© 

q 
e>i 

—  rooooooi-^eoooo 

crrrorr . ei 

io  •"  '*  <e  fcO  i*  I*  i*  io  »e 

o 

q 

os  — '  re  -o  _y  —  ei  -f  -.o  oo 

o eieirieiiM 

»e  »e  »e  »e  »e  <e  <e  »e  >e  «e 

O 

o 

2 

t~  c; u  -z  y  —  m  -f  -o 

.^  —  e  i  e  i  e  i  e  i  re  re  re  re 
•  e  ■".  to  «'e  '*e  <"e  >  e  io  >  e  10 

o 

o 

OS 

let^oscM-rftccr  —  ei  — 
ei  ei  ei  re  re  re  re  —  -f  h< 

= 

q 
oo 

K«NOfi-T  y  —  ri 

rerere-r-f-t<-«"^''eie 

IZ  >~.  '~.  '~  •'.   '~  '~  '~  '~  it} 

o 

q 

HMWOCOM-tOOOO 

■^  -p  -f  f  '--  io  io  »e  >e;  « 
io  Le  >e  *—  >—  ie  »—  »-e  i—  »— 

o 

q 

SO 

OS-HCOEOOOO<N^cOCO 

tj<  i-  i-  i-  i e  -o  o  o  co  ;o 
i—  »e  i—  »e  »e  >e  L.e  ic  ic  »o 

d 

q 
id 

ooos— i*aoooN'*!0 

WiOCCOcCcCNSNh. 

w3  io  »e  i.e  Le  Le  ue  io  »o  >c 

o 

q 

lOt^OSCN-^COOC  OcMt»< 

cOcDcOJ^t^t^t^oC'OOOO 

io  >-o  >o  io  *-e  io  »e  >e  o  io 

o 

q 

MONCN^OOOON 

r^i^i^oooooC'Xooosos 
io  »o  ic  i—  Le  i—  c  io  ie  io 

d 

o 

— <re>oaoocM-*cooo© 

oor/rccyjsscsoicscs© 
ic  ue  o  io  ie  Le  Le  te  ue  co 

© 

q 

i-H 

OS-HCOCDOOOtN-^cOOO 

0CCSO5C30SC©©©© 

i"  c  c  c  >e  cc  o  o  co 

© 

O 

© 

i^©— '-t<co30©e<»-*© 

OS©©©©©' — ?r^i— (^H 

»OiOCOCOOCOCOCO 

© 

13 

CP 

-r-r-r-f-r-r-t'TiiTiHid 
©©©©©cc©©©© 

198  BULLETIN  10",!).   U.   S.   DEPARTMENT  OF   AGRICULTURE. 

APPENDIX  B. 

T\ble   10—  Osmotic    pressure   in   atmospheres  for  depression   of   the  freezing    point 

to  2.999°  C.1 


0 

1 

2 

3 

4 

5 

6 

7 

8 

9 

0.0     

0.000 
1.206 
2.412 
3.616 
4.  821 

6.025 
7.229 
8. 432 
9.  635 
10.84 

12.04 

14.44 
15.64 
16.84 

1^.04 
19.24 
20.44 
21. 64 
22.  s4 

24.(14 
25.23 
26.43 
27. 63 

28.82 

0.121 
1.327 
2.532 
3.737 
4.941 

6. 145 

7.  349 

8.  552 

9.  7.").") 
10.96 

12.16 

13.  36 

14.  56 
15.76 
16.96 

18. 16 
19.36 
20.56 
21.76 
22.96 

24.16 
25. 35 

26.  55 

27.  75 

28.  94 

20.14 
31.33 

32.  55 

33.  72 
34.91 

0.241 
1.447 
2.652 
3.  S57 
5.062 

6.266 
7.469 
8.672 
9.  875 
11.08 

12.  28 

13.  Is 
14. 68 

15.  ss 
17.08 

18.28 
19.  48 
20.68 
21.88 
23.08 

24.23 
25.  47 
26.67 
27.87 
29.06 

20.26 
31.45 
32.65 
33.  84 
35.04 

0.362 
1.568 
2.  772 
3.978 
5.182 

6.386 
7.  590 
8.793 
9.995 
11.20 

12.  40 

13.  60 

14.  M) 
16.00 
17.20 

15.  40 
19.60 
20.  Ml 

22.  00 

23.  20 

24.  40 

25.  .7.1 

26.  79 
27.99 

29.  is 

30.  38 

31.  57 
32.77 
33.  96 
35.  16 

0.482 
1.688 
2.803 
4.09S 

5.  302 

6.  506 
7.710 
8.  913 

10.12 
11.32 

12.  52 

13.  72 
14.92 
16.12 
17.32 

1  v.  52 
19.  72 
20.92 
22.12 

23.  32 

24.  52 

25.  7 1 
26.91 
2s.  11 
29.  30 

30.50 
31.69 
32.  89 

34.  iiv 

35.  27 

0.603 
1.809 
3.014 

4.  219 

5.  423 

6.  628 

7.  830 
9.  033 

10.  24 
11.44 

12.6  4 
13.84 

15.(11 
16.21 
17.44 

18.  til 
19.84 
21.04 
22.24 

23.  1 1 

24.  63 

25.  83 
27.  03 
28.23 
29.42 

62 
31.81 
33.01 

34.  20 

35.  39 

0.724 
1. 930 
3.  134 
4. 339 

5.  .543 

6.747 
7.951 
9.  154 
10.36 
11.. 56 

12.76 
13.96 
15.  16 
16. 
17.56 

Is.  76 
19.96 
21.  16 
_'2.  36 
23.  .56 

24.75 
25.  95 
27.  15 
28.34 

29.  5 1 

30.74 
31.93 

13 
34.31 
35.51 

0.  S44 
2.050 

3.  25.', 

4.  459 
5.664 

6.- 

8.  071 

9.  27  4 
10.4v 
11.1 

12.  sv 
14.0V 
15.28 
16.48 

17.'.- 

is.  ^ 
20.08 
21.28 

22.  is 

23.  68 

24.87 
26.07 

27.  27 

28.  16 
29.66 

30.  - 

05 

33.  25 

34.43 

63 

0.965 

2.  171 

3.  375 
4.580 
5.784 

6. 988 
s.  191 
9.  394 
10.60 
11.80 

13.00 
14.20 
15.40 
16.60 

17.  so 

19.00 
20.20 
21.40 
22.60 
23.80 

24.  99 
26.19 

27.  39 

28.  5s 

29.  7s 

30.98 

17 
33. 

34. 
75 

1.085 

o-i 

0. 2          

2.291 
3.  496 

0.3 

0. 4     

4.700 
5.904 

ii.",                        

7.108 

i).  fi         

8.312 

n.  7             

9.514 

0.8 

0.9 

10.72 
11.92 

I:!:::::::::::::::::: 

1.2 

1.3 

13.  12 

14.  32 
15.52 
16.  72 

1.4 

17.92 

1.-". 

1.6 

1.7 

L.8 

1.9 

19.12 
20.32 
21.52 

22.  72 

23.  92 

2.0 

2.  1 

2.2 

2.3 

25.  1 1 

2d.  31 

27.  51 

28.  70 

2.4 

29.90 

2. 5 

31.09 

2.6 

2.7 

2.* 

31.21 
32.41 

33.  60 

34.  79 

32.29 

33.  is 
34.68 

2.9 

35.  s7 

1  Harris,  J.  A.,  and  Gortner,  R.  A.,  Amer.  Jour.  Botany,  1  :  75-78,  I'd  1. 

Table  11. — An  extension  to  .5.5.9°  of  tables  to  determint  the  osmotic  pressure  of  <  tpress  J 
vegetable  sups  from  tin  depression  of  the  freezing  point.1 

[Hundredths  of  degrees  centigrade.] 


3.0 

3.  1 

35.  99 
37. 18 

3.3 

38.38 
39.  57 

3.  4 

40  7»i 

3.5 

41.95 

3.7 

43. 14 
44  33 

3.8 

45  5'? 

3.9 

46  71 

4.0 

47.90 

4.1 

49  09 

4.2 

50  28 

4.3 

51  47 

4.4.... 

52  66 

4.5.... 

53  84 

4.6... 

55  03 

4.7... 

56  T> 

4.8.. 

57  40 

4.9.... 

5s  59 

5.0 

59  7s 

5.  1 . . . 

60  96 

5.2... 

62  14 

5.3... 

5.  4 . . . 

63.  33 
64  51 

5.5 

65  69 

...     

66   \s 

5.7... 

68  06 

5.8... 

69.24 
70.42 

5.9 

36.11 
37. 30 
38.50 
39.69 

40.88 

42.07 

43.  26 

44.  45 
45.64 
46.83 

48.02 
49.21 

50.  40 

51.  59 

52.  7s 

53.96 
55. 15 

56. 34 
57.  52 

5s.  71 

■V.i.  89 
61.  08 
62.26 

63.  15 

64.  63 

65.  si 
67.00 
68.  is 
69.36 

70.  54 


36.  23 

37.  42 
38.62 
39.  s] 
41.00 

42.19 
43.38 

44.  57 

45.  76 

46.  95 

48.  14 

49.  33 

50.  52 

51.  71 

52.  89 

54.08 

:>:>.  27 
56.46 

57.  64 

58.  83 

60.01 
61.20 

62.  38 

63.  56 

64.  75 

65.93 

67.  1 1 
68.30 
tilt.  48 
70.66 


36.  35 

37.  54 
3s.  73 
39.93 
41.12 

42.  31 
43.50 
44.69 
45.88 
47.07 


36.47 
37. 66 
38.  85 

40.  o:, 
41.24 

42.  43 

43.  62 

44.  si 
46.00 
47.19 


4s.  26 

18.    - 

49.  45 

lit.  57 

50.64 

50.  76 

51.83 

51.94 

53.01 

53.  13 

54.20 

.54.  32 

r^.  39 

55. 51 

56.  57 

56.  69 

57.  76 

57.  88 

5s.  95 

59.  06 

60. 13 

60. 

(11.32 

61.  43 

62.50 

62.62 

68 

63.80 

•  it. -7 

0  1.98 

66.05 

17 

•  17.  23 

(17.  35 

6s.  41 

•is.  53 

69.  60 

69.71 

70.  7s 

70. 

30.  5'.* 
37.  7^ 
38.97 
40.17 
41.36 

42.  55 

43.  74 

44.  93 

46.  12 
47.31 

48.50 

49.69 
50.88 
52. 06 

. 

54.  14 

55.  62 
56.81 

59.18 

0(1.37 
61.  55 
62.74 
63.92 
65.  I" 

67.  17 
ns.  65 
69.  83 

71.01 


37.  90 
39.09 
10.28 

11.  is 

42.  07 
32.  s., 
45.  Oo 
46.24 

47.  43 

4s.  62 

4-.'.  si 

.50.99 

52.  is 

1 

54.  56 

55.  7 1 

56.  93 

5s.  12 
59.30 

60.  49 

HI. 07 

64.  04 

65.  22 

66.  io 
07.  59 
6s.  77 
69.  95 

71.  13 


38.02 

40.  io 

41.00 

42.  79 

32.98 

45.  17 

47.  55 

4s.  71 
49.  93 
51.  11 

52.30 
53.  i'.i 

57.0.". 

61.79 
64.  16 

07.71 

70.07 
71.25 


38.  1 ) 

40.52 
41.71 

42.91 
44.10 
45.  29 

Hi.  lv 
17.  07 

is.  Vii 

51.23 

54.  7'.i 
57.  17 

61.91 
64.27 

H5.  I.. 
71.37 


37.00 

>.  l". 
Ki.  HI 
41.83 

13.1(2 

14.  22 

15.  41 

16.60 

17.  79 

4s.  97 
50.  16 
5L35 

■ 

54.91 

56.  10 

58.  47 

M 

63.21 

H4.39 

I 
71.  49 


1  Harris,  J.  A.,  Amer.  Jour,  of  Botany,  2:  418-419.  1915. 


RESEARCH  METHODS  IX  STUDY  OF  FOREST  ENVIRONMENT.      199 

APPENDIX  C. 

STANDARD  TITRATION  METHODS  FOR  SOIL  ACIDITY  AND  FOR  CARBONATES 

("BLACK  ALKALI"). 

The  following  procedure  in  titration  for  the  alkalinity  and  acidity  tests  is  practically 
that  followed  by  the  Bureau  of  Soils,  and  a  number  of  soil  departments  in  the  agricul- 
tural experiment  stations: 

The  equipment  required  x  is  two  50-cubic  centimeter  burette  titration  apparatuses, 
one  50-cubic  centimeter  graduate,  one  250-cubic  centimeter  graduate,  two  50-cubic 
centimeter  Xesslar  tubes,  1-liter  flask,  four  100-cubic  centimeter  beakers,  two  50-cubic 
centimeters  Royal  Berlin  porcelain  evaporating  dishes,  one  50-cubic  centimeter 
pipette,  two  ordinary  pipettes  or  droppers,  bottles  and  jars  for  reagents,  analytical 
balance,  numerous  quart  jars  with  screw  caps  or  stoppers,  and  reagents  as  indicated 
by  the  procedure.     The  necessary  reagents  are  prepared  as  follows: 

(1)  Standard  potassium  hydrogen  sulphate  solution: 

The  average  single  test  will  not  require  over  5  cubic  centimeters  of  this  solution. 
Dissolve  5.58  grams  of  pure  KHS04  in  1  liter  of  water,  and  dilute  100  cubic  centi- 
meters of  this  solution  to  1  liter.  Place  the  dilute  solution  in  burette  jar  Xo.  1, 
for  alkalinity  titrations. 

(2)  Phenolphthalein  indicator: 

A  drop  or  two  for  each  alkalinity  and  acidity  test  is  required.  Dissolve  1  gram 
of  phenolphthalein  in  100  cubic  centimeters  of  50  per  cent  alcohol.  Neutralize 
by  adding  a  few  drops  of  centinormal  alkali,  until  faintly  red,  then  add  a  drop 
of  centinormal  acid,  which  should  remove  the  color. 

(3)  Methyl  orange  indicator: 

A  drop  or  two  for  each  alkalinity  test  is  required.  Dissolve  1  gram  of  methyl 
orange  (indicator)  in  1  liter  distilled  water. 

(4)  Normal  alkali  is  prepared  by  dissolving  39.96  mams  of  NaOH  in  1  liter  of  water. 
Since  only  a  few  drops  of  the  centinormal  solution  are  needed  in  preparing  the 

phenolphthalein  indicator,  and  the  exact  strength  is  unimportant,  use  in  about 
the  proportion  of  0.04  gram  per  100  cubic  centimeters  of  water. 

(5)  Centinormal  acid  (HCl): 

Exact  strength  unimportant.  About  2  drops  in  100  cubic  centimeters  of  water 
give  approximately  correct  strength. 

(6)  Standard  sodium  hydroxide  solution: 

Compute  quantity  required  at  rate  of  one-half  to  1  cubic  centimeter  per  acidity 
test  made.  The  solution  is  not  normal,  but  is  computed  so  that  1  cubic  centi- 
meter will  have  the  equivalent  value  of  4  mg.  of  calcium.  Dissolve  6.4  grams  of 
pure  NaOH  in  1  liter  of  freshly  boiled  distilled  water.  Place  this  in  burette  jar 
Xo.  2,  for  acidity  titrations.  Exclude  air  from  the  jar  as  far  as  possible  and  make 
up  fresh  solution  frequently. 

(7)  Normal  potassium  nitrate  solution: 

Use  250  cubic  centimeters  for  each  soil  examined  for  acidity.  Dissolve  100.93 
grams  of  pure  KN03  in  1  liter  of  distilled  water. 

Alkalinity  Test. 

Place  100  grains  of  air-dried  soil  of  the  sample  to  be  examined  in  a  quart  jar;  add 
200  cubic  centimeters  of  distilled  water:  shake  occasionally  during  12  hours  and  allow 
to  settle.  The  test  may  best  be  started  early  in  the  day,  shaking  jars  occasionally  dur- 
ing the  day  and  leaving  them  to  settle  overnight.  Turbidity  of  the  solution  is  difficult 
to  eliminate  in  this  test,  but  time  is  saved  if  complete  settling  occurs. 

Draw  off  with  pipette  50  cubic  centimeters  of  the  supernatent  liquid;  filter,  if 
not  fairly  clear,  into  an  evaporating  dish;  and  evaporate  over  Bunsen  flame,  continu- 
ing the  drying  almost  to  red  heat,  so  that  residue  is  devoid  of  humus  and  will  cling 
together  in  flakes  when  the  dish  is  scraped.  When  dish  is  cooled  add  50  cubic  centi- 
meters of  distilled  water:  allow  to  stand  for  two  hours;  then  pour  half  of  solution  into 
each  of  two  50-cubic  centimeter  Xesslar  tubes.     If  fusing  of  residue  has  been  complete, 

1  The  special  equipment  required  to  conduct  the  work  at  experiment  stations  should  cost  approximately 
J20  to  S2.i. 


200  BULLETIN  1059,  U.   S.   DEPARTMENT  OF  AGRICULTURE. 


filtering  at  this  stage  would  be  unnecessary,  as  the  flakes  of  solid  matter  will  remain  in 

evaporating  dish. 

Add  to  each  Nesslar  tube  a  drop  of  phenolphthalein  indicator,  comparing  the  first 
rube  with  that  which  has  not  been  treated  before  treating  the  second.  If  the  solu- 
tion in  tube  1  is  colored  pink,  carbonate  (Na2C03)  is  present,  and  the  solution  is  ti- 
trated with  KIISO,  until  pink  color  disappears.  The  burette,  presumably  graduated 
to  tenths  of  a  cubic  centimeter,  should  be  estimated  to  the  nearest  hundredth  just 
1  tefore  beginning  to  titrate,  and  again  as  soon  as  the  color  disappears.  The  comparison 
tube  may  then  be  similarly  treated,  reading  the  burette  for  the  second  treatment,  as 
a  check  on  the  first.     This  second  tube  is  carried  simply  as  a  color  comparator. 

A  drop  of  methyl  orange  indicator  is  now  added  to  each  of  the  above  tubes,  whether  or 
not  the  titration  for  carbonate  has  been  made.  This  will  give  to  each  a  yellow  color, 
and  the  second  tube  will  be  kept  alongside  the  one  which  is  titrated  so  that  change  of 
color  in  the  latter  will  be  quickly  apparent.  The  titration  of  the  first  tube  is  con- 
tinued until  the  slightest  reddish  or  orange  tinge  appears.  This  may  besl  be  discerned 
against  a  white  background  and  not  in  direct  sunlight,  which  sometimes  Ltself  imparts 
a  reddish  tinge  to  the  yellowish  solution.  A  final  reading  of  the  burette  is  obtained  to 
compute  the  quantity  of  KHS04  used  in  titrating  for  the  bicarbonate  (NaHCM  \ 

The  amount  of  the  second  titration,  less  the  amount  of  the  first,  gives  the  amount 
necessary  to  neutralize  the  HC03  originally  present,  since  the  firsl  reaction  changed  the 
carbonate  to  bicarbonate,  as  follows:  Xa2C034-KHS04=K\aS<  >4-f-NaHCOs. 

The  second  titration,  or  the  first  if  no  carbonate  was  originally  present,  reduces  all 
bicarbonate  present  to  carbonic  acid,  as  shown  by  t  he  reaction: 

NaHC03+KHS04=KNaS04+HC2<  >3. 

For  each  0.01  cubic  centimeter  of  KHS04  used  in  the  firsl  titration,  there  was  present 
0.00246  milligram  of  C03  in  the  25-cubic  centimeter  solution  treated,  or  0.0196S  milli- 
gram in  the  solution  representing  100  grams  of  soil,  or  0.00001968  per  cent  of  the  soil 
weight. 

For  each  0.01  cubic  centimeter  of  KHS04  in  the  differential  titration,  there  was 
present  in  the  original  solution  0.0025  mg.  of  HC03  in  the  25-cubic  centimeter  solution 
used,  or  0.02  mg.  in  the  solution  representing  100  grams  of  soil,  or  0.00002  per  cent  of 
the  weight  of  the  soil. 

Acidity  Test. 

Place  100  grams  of  air-dried  soil  of  the  sample  to  be  tested  in  a  quart  jar :  add  250  cubic 
centimeters  of  normal  KN03  solution  and  stopper:  and  shake  at  intervals  of  5  minutes 
for  3  hours.  Let  stand  overnight.  Draw  off  125  cubic  centimeters  of  the  supernatant 
liquid,  which  in  this  case  is  usually  quite  clear;  boil  10  minutes  to  expel  carbon  diox- 
ide: cool,  and  add  a  drop  of  phenolphthalein  indicator.  Place  the  beaker  under  the 
burette,  against  a  white  background,  and  titrate  with  XaOH  to  the  appearance  of  the 
faintest  pink   color. 

The  amount  of  the  titration  being  determined  by  readings  of  the  burette,  the  amount 
of  acid  present  in  the  solution  is  expressed  by  the  amount  of  lime  which  would  be 
required  to  neutralize  it.  Each  0.01  cubic  centimeter  of  the  sodium  hydroxide  used 
in  titration  is  equivalent  to  0.04  mg.  of  calcium  carbonate  in  the  125-cubic  centimeter 
solution  used,  and  while  this  stands  for  one-half  of  the  100  -rams  of  the  soil  treated,  it 
really  stands  for  only  two-fifths  of  the  acid  in  that  soil,  because  the  first  solution  does 
not  completely  dissolve  the  acids.  Therefore,  each  0.01  cubic  centimeter  of  titration 
indicates  0.1  mg.  of  lime  necessary  to  neutralize  the  100  grams  of  soil,  or  the  amount 
required  to  neutralize  is  0.0001  per  cent  of  soil  weight. 

In  practice,  the  amount  of  lime  required  to  neutralize  the  firsl  foot  ofsoil  is  expressed 
in  tons  per  acre,  being  computed,  of  course,  on  a  standard  or  specific  weighl  ofsoil  per 
acre-foot. 

Space  is  provided  on  "Summary  of  Physical  and  Chemical  Properties  of  Soil  "  form 
for  tabulating  the  computed  results  of  alkalinity  and  acidity  teste  in  terms  of  pi 
centages  of  the  weight  ofsoil,  which  are  as  serviceable  for  scientific  comparisons  as  anv 

other  expression. 


LIST   OF  REFERENCES. 

The  following  citations  to  the  literature  of  forest  ecology  are  mainly  those  con- 
cerned with  methodology.  A  few  references  are  given  to  descriptive  works  in  which 
the  methods  of  obtaining  the  results  are  clearly  brought  out,  or  in  which  the  nature 
of  the  problem  to  lie  met  by  the  future  ecologist  is  emphasized.  No  attempt  has 
been  made  to  prepare  a  complete  bibliography,  and  the  convenience  of  the  average 
student  has  received  considerable  weight,  in  avoiding,  especially,  foreign-language 
articles. 

<;i:\  ERAL. 

1.  Abbe,   Cleveland.     Treatise  on  meteorological  apparatus  and  methods.     An- 

nual Report  of  the  Chief  Signal  Officer  for  1887,  Appendix  46,  Signal 
Service.  War  Dept..  Washington.  L888. 

2.  Bates,  C.  G.,  Notestein,  F.  &.,  and  Keplixger,  P.     Climatic  characteristics 

of  forest  types,  in  the  Central  Rocky  Mountains.     Proc.  Soc.  Am.  For- 
esters, IX,  1,  Wash.,  1914. 

3.  Bigelow,  F.  H.    Manual  for  observers  in  climatology  and  evaporation.     U.  S. 

Weather  Bur.,  1909,  pp.  106. 

4.  Boerker,  R.  H.     Some  notes  on  forest  ecology  and  its  problems.     Proc.  Soc. 

Am.  Foresters,  X,  4,  Washington,  1915. 

5.  Bowman,  1.     Forest  physiography.     Xew  York,  1911. 

6.  Clements,  F.  E.     Research  methods  in  ecology.     Lincoln,  Nebr.,  1905. 

7.  Plant  physiology  and  ecology.     Xew  York,  1907. 

8.  Haxx,  Julius.     Handbook  of  climatology.     (Transl.  by  R.  deC.  Ward.)     Xew 

York,  pp.  437,  1903. 

9.  Harrixgtox,   M.   W.     Review  of  forest  meteorological  observations:   a  study 

preliminary  to  the  discussion  of  trfe  relation  of  forests  to  climate.     U.S. 
Forest  Serv.,  Bull.  7,  1893. 

10.  Harriot,  Wm.     Hints  to  meteorological  observers.     Royal  Meteorological  Soc, 

London,  1911. 

11.  Moore,  W.  L.     Descriptive  meteorology.     Xew  York,  pp.  344,  1910. 

12.  Pearsox,  G.  A.     Reproduction  of  western  yellow  pine  in  the  Southwest.     L~.  S. 

Dept.  Agr.,  Forest  Serv.,  Circular  174,  1910. 

13.  Meteorological  study  of  parks  and  timbered  areas  in  the  western  yellow 

pine  forests  of  Arizona  and  Xew  Mexico.     U.  S.  Weather  Bur.,  Mo. 
Weather  Rev.,  XLI,  pp.  1615-1629,  1913. 

14.  Factors  controlling  the  distribution  of  forest  types.     Ecology,  I,  3,  1920. 

15.  Schimper,  A.  F.  W.     Plant  geography  upon  a  physiological  basis.     (Transl.  by 

W.  R.  Fisher),  Oxford,  pp.  839,  1903. 

16.  Shreve,  Forrest.     The  vegetation  of  a  desert  mountain  range  as  conditioned 

by  climatic  factors.     Carnegie  Institution,  Washington,  1915. 

17.  Warmixg,  Eug.     Ecology  of  plants.     (Transl.  by  Groom  and  Balfour.)    Oxford, 

pp.422,  1909. 

201 


_    12  BULLETIN   1010,  U.   S.   DEPARTMENT  OF  AGRICULTURE. 

is    Weather  Bureau.     U.  S.  Climatological  data  of  the  United  States.      A  monthly 

record  by  sections— States— issued  since  Jan.,  1914,  mainly  of  tempera- 
ture and  precipitation  data  for  each  station  reporting  to  the  Weather 
Bureau.  Prior  to  L914,  all  similar  data  were  published  in  the  Monthly 
Weather  Review.  Current  conditions  are  given,  as  well  as  variations 
from  the  normal  for  stations  established  10  years  or  mor< 
19.  Zon,  Raphael.  Meteorological  observations  in  connection  with  botanical 
geography,  agriculture,  and  forestry.  U.  S.  Weather  Bureau,  Mo. 
Weather  Rev.,  XLII,  4,  1914. 

AIR    TEMPERATURES. 

21.  Fassig,  0.  L.     Period  of  safe  plant  growth  in  Maryland  and  Delaware.     U.  S. 

Weather  Bureau,  Mo.  Weather  Rev.,  XL,  3,  1914. 

22.  Hartzell,  F.  Z.     Comparison  of  methods  for  computing  daily  moan  tempera- 

tures: effect  of  discrepancies  upon  investigations  of  climatologists  and 
biologists.  N.  Y.  Agr.  Exp.  Station  Bull.  68,  L919.  Abstracl  in  Mo. 
Weather  Rev.,  p.  799,  Nov.,  1919. 

23.  Koeppen,  Vladimar.     A  uniform  thermometer  exposure  at  i Meteorological  sta- 

tions for  determining  air  temperatures  and  atmospheric  humidity. 
U.  S.  Weather  Bureau.  Mo.  Weather  Rev.,  XLIII,  8,  L915. 

24.  Lehexbauer,   P.   A.     Growth  of  maize  seedlings  in   relation  to  temperature. 

Physiological  Researches,  I,  5,  pp.  247-288,  Baltimore,  L914. 

25.  Livingston,  B.  E.,  and  Livingston,  G.  J.     Temperature  coefficients  in  plant 

geography  and  climatology.     Bot.  Caz.  56,  pp.  349  375,  L913. 

26.  Livingston,  B.  E.     Physiological  temperature  indices  for  the  study  of  plant 

growth  in  relation  to  climatic  conditions.  Physiological  Researches, 
1:8,  399-420,  1916. 

27.  MacDougal,  D.  T.     The  auxothermal  integration  of  climatic  complexes.     Amer. 

Jour.  Bot.,  I  :  186-193,  1914. 
2£    Marvin,  C.  F.     Instructions  for  obtaining  and  tabulating  records  from  recording 
instruments.     U.  S.  Weather  Bureau,  Circular  A,  Instrumenl  Div.,  L913. 

29.  -  Sluggishness  of  thermometers.     V .  S.  Weather  Bureau,  Mo.  Weather  Rev.i 

XXVII,  10,  1899. 

30.  Merriam,  C.  Hart.     Life  zones  and  temperature  zones.      L898 

31.  McLane,   F.   T.     A  preliminary  study  of  climatic  conditions  in  Maryland  as 

related  to  plant  growth.  Physiological  Researches  14.  (Baltimore) 
February,  1917. 

32.  Sampson,  A.  W.     Climate  and  plant  growth  in  certain  vegetative  associations. 

I'.  S.  Dept.  Agr.  Bull.  Xo.  700,  1918. 

33.  Seeley,  D.  A.     Instruments  for  making  weather  observations  on  the  farm.     U.  S 

Dept.  Agr.  Yearbook,  1908,  pp.  433-442. 

34.  Shreve,  Forrest.     The  influence  of  low  temperatures  on  the  distribution  of 

the  giant  cactus.     Plant  World,  14,  6,  191 1 . 

35. Cold  air  drainage.     Plant  World.  15,  5,  1912. 

36.  Shreve,   Edith  B.     Thermo-electrical   method   for  the   determination   of   leaf 

temperature.     Plant  World,  22,  6,  1919. 

37.  Standards,  U.  S.  Bur.  of.    Testing  of  thermometers.     Circ.  No.  8,  1911. 

38.  Thiessen,  A.  H.     Story  of  the  thermometer  and  its  uses  in  agriculture.     I      - 

Dept.  Agr.  Yearbook,  1914,  pp.  458-461. 

39.  Weather  Bureau,  U.  S.     Instructions  for  cooperative  observers  of  the  Weather 

Bureau.     Circs.  B  and  C,  Instrument  Div.,  1915. 


RESEARCH  METHODS  IN  STUDY  OF  FOREST  ENVIRONMENT.      203 

SOIL  TEMPERATURES. 

40.  Abbe,  Cleveland.     First  report  on  the  relations  between  climate  and  crops. 

U.  S.  Weather  Bur.  Bull.  36,  1905. 

41.  Bouyoucos,   G.   J.     Effect  of  temperature  on  the  movement  of  water  vapor 

and  capillary  moisture  in  soils.     U.   S.   Dept.   Agr.,  Jour.   Agr.   Res., 
V,  4,  1915. 

42.  Soil  temperature.     Mich.  Agr.  Exp.  Station  Tech.  Bull.  26,  1916. 

43.  Ferrel,  Wm.     Temperature  of  the  atmosphere  and  the  earth's  surface.     U.  S. 

Signal  Serv.,  War.  Dept.,  Prof.  Paper  13,  1884. 

44.  Hartley,  Carl.     Stem  lesions  caused  by  excessive  heat.     U.   S.   Dept.   Agr., 

Jour.  Agr.  Research,  XIV,  13,  1918. 

45.  MacDougal,  D.  T.     Soil  temperature  and  vegetation.     U.  S.  Weather  Bureau 

Mo.  Weather  Rev.,  XXXI,  8,  1903. 

46.  Oscamp,  J.     Soil  temperatures  as  influenced  by  cultural  methods.     U.  S.  Dept. 

Agr.,  Jour.  Agr.  Research,  V,  4,  1915. 

47.  Patten.    II.  E.     Heat  transference  in  soils.     U.   S.  Dept.   Agr.,  Bur.  of  Soils, 

Bull.  59,  L909. 

48.  Seeley,    D.    A.     Temperature  of  the  soil  and  surface  of  the  ground.     U.   S. 

Weather  Bureau  Mo.  Weather  Rev..  XXIX.  II,  1901. 

SOLAK    RADIATION  —LIGHT. 

50.  Abbot,  C.  G.,  and  Aldhich.  L.  B.     Smithsonian  pyrheliometry  revised.     Smith- 

sonian Misc.  Coll.,  vol.  60,  Xo.  18,  1913. 

o 

51.  Angstrom,  A.     A  new  instrument  for  measuring  sky  radiation.     U.  S.  Weather 

Bureau  Mo.  Weather  Rev.,  XLVII,  pp.  795-797,  Nov.,  1919. 

52.  Baly,   E.  C.  C.     Spectroscopy.     Pp.  568,  Longmans.   London  and  Xew  York, 

1905. 

53.  Bigelow,    F.    II.     Mr.   Abbot's  theory  of  the  pyrheliometer.     Science,  n.   s., 

vol.  XLVIII,  Oct.  25,  L918. 

54.  Briggs,    L.    J.     A    mechanical    differential   telethermograph    and    some   of   its 

applications.     Jour.  Wash.  Acad.  Science,  vol.  3,  Xo.  2,  1913. 

55.  Bunsen,   K.  and    Roscoe  II.     Meteorologische   lichtmessungen.     Poggendorff's 

Annalen,  vol.   117,  1862. 
5"i\.   Be kn's,  G.  P.     Studies  on  the  tolerance  of  Xew  England  forest  trees. 

56. I.  Development   of    white    pine   seedlings   in    nursery   beds.     Vt.    Agr. 

Exp.  Sta.  Bull.  178,  1914. 

57.  -  II.  Relation  of  shade  to  evaporation  and  transpiration  in  nursery  beds. 

Vt.  Agr.  Exp.  Sta.  Bull.  181,  1914. 

58.  ■  III.  Discontinuous  light  in  forests.     Vt.  Agr.  Exp.  Sta.  Bull.  193,  1916. 

59.  Tolerance  of  forest  trees  and  its  relation  to  forest  succession.     Jour,  of 

Forestry,  XVIII,  6,  1920. 

60.  Clements,  F.  E.     The  life  history  of  lodgepole  burn  forests.     U.  S.  Forest  Serv. 

Bull.  79,  1910. 

o  t 

61.  Davis,  Harvey  N.     Observations  of  solar  radiation  with  Angstrom  pyrheliom- 

eter.    Providence,    R.    I.,   Mo.    Weather  Rev.,  XXXI,  6,  pp.  275-280, 
June,  1903. 

o 

62.  Kimball,  H.  H.     Observations  of  solar  radiation  with  Angstrom  pyrheliometer, 

Asheville  and   Black  Mountain,    N.    C.     U.    S.    Weather   Bureau  Mo. 
Weather  Rev.,  XXXI,  7,  pp.  320-334,  July,  1903. 

63.  Photometric    measurements   of   daylight   illumination    on    a  horizontal 

surface  at  Mount  Weather,  Va.     U.  S.  Weather   Bureau  Mo.  Weather 
Rev.,  XLII,  pp.  650-653,  1914. 


PRECIPITATION- 


SI.  Abbe,  Cleveland.     Rain  gage  and  the  wind.     (Ed.  Notes,  Mo.  Weather  Rev., 
XXVII,  10,  p.  464-468,  Oct.,  1899. 

82.  Alter,  J.  C.     Where  the  snow  lies  in  summer.     TJ.   S.   Weather   Bureau  Mo. 

Weather  Rev.,  XXXIX,  pp.  758-761,  15,  May,  1911. 

83.  Beals,  E.  A.     Variations  in  rainfall.     Mo.  Weather  Rev.,   XXIX.  9,  p.   1448- 

1452,  Sept.,  1911. 

84.  Big  flow,   F.  II.     Catchment   of   snowfall    by  means  of    large  snow    bins  and 

towers.     Mo.  Weather  Rev..  XXXVIII,  6.  p.  968  973,  June,  1910. 


* 


204         BULLETIN   1059,   U.    S.   DEPARTMENT    OF   AGRICULTURE. 

64  Kim  hall,  H.  H.  Total  radiation  received  on  horizontal  surface  from  sun  and  sky 
at  Mount  Weather.  Va,  U.  S.  Weather  Bureau  Mo.  Weather  Rev., 
XLII,  8,  Aug.,  1914,  pp.  474-187,  and  XLIII,  pp.  100-111.  Mar.,  1915. 

Variation' in  the  total   and   luminous  solar  radiation  with  geographical 

position  in  the  United  States.     U.   S.  Weather  Bureau  Mo.   Weather 
Rev.,  XLVII,  pp.  769-793,  Nov.,  1919. 
r,  C.  G.     Solar  radiation  and  earth  temperatures.     U.  S.  Weather  Bureau 
Mo.  Weather  Rev..  XXXI,  10,  pp.  454-459,  Oct.,  1903. 

67.  Langley,  S.  P.     Researches  on   solar  heat  and  its  absorption   by  the  earth's 

atmosphere.  (Mount  Whitney  Expedition.)  Papers  of  the  Signal  Service, 
Xo.  15.  War  Dept.,  1884. 

68.  MacDougal,  D.  T.,  and  Spoehr,  H.  A.     The  measuremenl   of  light  in  some 

of  its  more  important  physiological  aspects.  Science,  u.  s.,  vol.  XLV, 
Xo.  1172,  June  15,  1917. 

69.  Marvin,  C.  F.     The  measurement  of  sunshine  and  the  preliminary  examination 

of  Angstrom's  pyrheliometer.  U.  S.  Weather  Bureau  Mo.  Weather 
Rev.,  XXIX.  Oct.,  1901. 

70.  Care  and  management  of  sunshine  recorders.     3d  ed..  22  p.       circular 

G,  Instrument  Div.),  U.  S.  Weather  Bureau,  1911. 

71.  Poynting,  J.  H.     Radiation  in  the  solar  system.     D     S     Weather  Bureau  Mo. 

Weather  Rev..  XXXII.  11,  pp.  507  511,  Nov..  L904. 

72.  Radau.     Actinometrie.     Paris,   L877. 

73.  Sharp,  C.  H.,  and  Miller,  P.  S.     A  new  universal  photometer.     Electrician, 

60,  pp.  562-565,  1908. 

74.  Very,    Frank   W.     Atmospheric   radiation.     (A    research    conducted    at    Alle- 

gheny Observatory  and  at  Providence,  R.  I  L34  p.  U .  S.  Dept. 
of  Agr.,  Weather  Bureau  Bull.  G.,  L900. 

75.  —  The  solar  constant.     U.  S.  Weather  Bureau  Mo.  Weather  Rev.,  XXI. X.  j 

Aug.,  1901. 

76.  Weisner,  Julius.     Orientirende  Versuche  uber  den   Einfluss  der  aogenannten 

chemischen  Lichtintensitat  auf  den  Gestaltungsprocees  der  Pflan- 
zenorgane.  (In  Sitzungsberichte  der  K.  Akademie  der  Wissenschaften, 
Wien,  Mathematische-naturwissenschaftliche  Klasse,  vol.  102,  pt.  1  :291- 
350,  1893.) 

77.  Zox.  Raphael.     A  new  explanation  of  the  tolerance  and  intolerance  of  trees. 

Proc.  Soc.  Am.  Foresters,  II,  1.  1907. 
78. and  Graves,  H.  S.     Light  in  relation  to  tree  growth.     V .  S.  Forest  Serv. 

Bull.  92,  1911. 
79.  Zederbauer,  Emerich.     Das  Lichtbediirfniss  der  Waldbaume  und  die  Lieht- 

mess  Methoden.     (In  Centralblatt  fur  das  gesamte   Forstwesen,    L907, 

vol.  33  :  325-330.     Transl.  in  Forestry  Quarterly,  vol.  6  :  255-262,  1908.) 


RESEARCH  METHODS  IX  STUDY  OF  FOREST  ENVIRONMENT.      205 

85.  Church,  J.  E.  Jr.     The  conservation  of  snow;  its  dependence  on  mountains  and 

forests.     Bull,  of  the  University  of  Nev.,  Agr.  Exp.   Station,  vol.   1 
Xo.  6,  Dec,  1912. 

86.  .     The  progress  of  Mount  Rose  Observatory,   1906-1912.     Science,  n.  s., 

Vol.  XXXVI,  No.  939,  December,  1912. 

87.  .     The    Mount    Rose   weather    observatory.     1906-1907.     Bull.    No.    67. 

University  of  Nev.  Agr.  Exp.  Station. 

88.  .     Snow  survey  provides  basis  for  close  forecast  of  watersheds'   yield. 

Engineering  Record,  April  17,  1915. 

89.  Hortox,  R.  E.     Rainfall  interception.     U.  S.  Weather  Bureau,  Mo.  Weather 

Rev.,  XLVII,  9,  Sept.,  1919. 

90.  Jaenicke,  A.  J.,  and  Foerster,  M.   H.     The  influence  of  a  western  yellow- 

pine  forest  on  the  accumulation  and  melting  of  snow.     U.  S.  Weather 
Bureau,  Mo.  Weather  Rev.,  p.  115-26,  March,  1915. 

91.  Kadel,  B.  0.     An  improved  form  of  snow  sampler.     U.  S.  Weather  Bureau, 

M<>.  Weather  Rev..  XLVII,  10,  Oct.,  1919. 

92.  Kincer,   J.   B.     The  seasonal  distribution  of  precipitation  and  its  frequency 

and  intensity  in  the  United  States.     U.  S.  Weather  Bureau,  Mo.  Weather 
Rev.,  XLVII,  9,  Sept.,  1919. 

93.  Marvin,   C.   F.     The  measurement  of  precipitation;  instructions  on  measure- 

ment and  registration  of  precipitation  by  means  of  standard  instruments 
of  Weather  Bureau.     3d  ed.,  Instrument  Div.  Circ.  E,  37  p.,  1913. 

94.  Stockman",  W.  B.     Periodic  variation  of  rainfall  in  arid  region.     15  p.,  Weather 

Bur.  Bull.  N,  1905. 

95.  Thiessex,  A.  H.     Value  of  snow  surveys  as  related  to  irrigation  projects,  p. 

391-396,  U.  S.  Dept,  Agr.,  Yearbook  1911,  separate  578. 

96.  U.  S.  Weather  Bureau.     Instructions  to  special  river  and  rainfall  observers  of  the 

Weather  Bureau.     U.  S.  Dept,  Agr.,  Weather  Bull.  No.  415,  1909. 

97.  .     Snow  and  ice  bulletin.     (^Weekly  during  winter.) 

98.  Wallis,  B.  C.     Rainfall  and  agriculture  in  United  States.     Mo.  Weather  Rev., 

XLIII,  6,  p.  267-274,  map.     June,  1915. 

SOILS. 

101.  Alway,  F.  J.,  Files,  E.  K.,  and  Pinckney,  R.  M.     The  determination  of  humus. 

Nebr.  Agr.  Exp.  Station  Bull.  115,  1910. 

102.  .     Studies  of  the  relation  of  the  nonavailable  water  of  the  soil  to  the  hygro- 

scopic coefficient.     Nebr.  Agr.  Exp.  Station,  Research  Bull.  3,  1913. 

103.  .     Kline,  M.  A.,  and  McDole,  G.  R.     Some  notes  on  the  direct  deter- 

mination of  the  hygroscopic  coefficient.     U.  S.  Dept.  Agr.,  Jour.  Agr. 
Research,  XI,  4,  October  22,  1917. 

104.  Bates,  C.  G.     Concerning   site.     Jour,    of    Forestry,  XVI,  4.     (A  suggestion 

as  to  factors  controlling  height  growth.)     April,  1918. 

105.  .     The  descriptions  and  data  given  under  this  reference  are  original  con- 

tributions resulting  from  studies  at  the  Fremont  Experiment  Station, 
from  1914  to  date,  heretofore  unpublished;  hence  given  in  detail. 

106.  Bouyoucos,  G.  J.,  and  McCool,  M.  M.     The  freezing-point  method  as  a  new 

means  of  measuring  the  concentration  of  the  soil  solution  directly  in  the 
soil.     Mich.  Agr.  Exp.  Station,  Tech.  Bull.  24,  1915. 

107.  .     .     Further  studies  on  the  freezing-point  lowering  of  soils.     Mich. 

Agr.  Exp.  Station,  Tech.  Bull.  31,  1916. 

108.  .     The  freezing-point  method  as  a  new  means  of  determining  the  nature 

of  acidity  and  lime  requirements  of  soils.     Mich.  Agr.   Exp.   Station 
Tech.  Bull.  27,  1916. 


206  BULLETIN    105!),    U.    S.   DEPARTMENT   OF   AGRICULTURE. 

109.  Bouyoucos,  G.  J.,  and  McCool,  M.  M.     Measurement  of  inactive,  or  unfi 

moisture  in  the  soil  by  means  of  the  dilatometer  method.     U.  S.  Dept. 
Agr.3  Jour.  Agr.  Research,  VIII,  6,  Feb.,  1917. 

1 10.  Briggs,  L.  J.     The  mechanics  of  soil  moisture.     U.  S.  Dept.  Agr.,  Bu.  of  Soils, 

Bull.  Xo.  10,  1897. 

J II.  Briggs,  L.  J.  Electrical  instruments  for  determining  the  moisture  tempera- 
ture, and  soluble  salt  content  of  soils.  U.  S.  Dept.  Agr.,  Bu.  of  Soils, 
Hull.  L5,  1899. 

L12. .  Martix,  F.  O.,  and  Pearce.  J.  R.  The  centrifugal  method  of  me- 
chanical soil  analysis.     U.  S.  Dept.  Agr.,  Bur.  of  Soils,  Bull.  24,  1904. 

1  L3.  and  McLane,  J.  W.    The  moisture  equivalents  of  soils.     V.  S.  Dept    Agr., 

Bur.  of  Soils,  Bull.  45,  1907. 

114 ancl   Shantz,   H.    L.     The  wilting  coefficient  for  different   plants  and 

its  indirect  determination.     U.  S.  Dept.  Agr.,   Bur.  of  Plant  Industry, 
Bull.  230,  1912. 

II").   Buckingham,  E.     Contributions  to  our  knowledge  of  the  aeration  of  soils.     U.  S 
Dept,  Agr.,  Bur.  of  Soils,  Bull.  25,  1904. 

116.  .     Studies  on  the  movement  of  soil  moisture.     U.  S.  Dept    Agr.,  Bur.  of 

Soils,  Bull.  No.  38,  1907. 

117.  Cameron,  F.  K.,  and  Gallagher,  F.  E.     Moisture  content  and  physical  con- 

ditions of  soils.     U.  S.  Dept.  Agr..  Bur.  of  Soils,  Bull.  50,  l> 

118.  Chemistry,  Bureau  of.     Official  and  provisional  methods  of  anal;  sis      I      S. 

Dept.  Agr.,  Bull.  107. 

119.  Dixon,  H.  H.,  and  Atkins,  W.  R.  G.     Osmotic  pressures  in  plants.     I.   Method 

of  extracting  sap  from  plant  organs.     Sci.   Proc.   Royal  Dublin   Soc, 
n.  s.  13,  pp.  422-33,  1913. 

120.  Fletcher,  C.  C,  and  Bryan,  H.     Modification  of  the  method  of  mechanical 

soil  analysis.     U.  S.  Dept.  Agr.,  Bur.  of  Soils,  Bull.  84,  1912. 

121.  Free,  E.  E.     Studies  in  soil  physics.  The  Plant  World,  v.    II.  Nos.  2.  3,  5,  7. 

and  8,  1911. 

122.  Grandeau,  Louis.     Traite'  d'Analyse  des  Matieres  agricoles,  Paris,  ls77. 

123.  Harris,  J.  A.,  Lawrence,  J.  V.,  and  Gortner,  R.  A.     The  cryoscopic  con- 

stants of  expressed  vegetable  saps  as  related   to  local   environmental 
conditions  in  the  Arizona  deserts.     Phy.  Res.  vol.  2,  No.  I.  L916. 

124.  Hartley,   Carl.     The  control  of  damping-off  of  coniferous   seedlings.     Bur. 

Plant  Ind.,  U.  S.  Dept.  Agr.  Bull.  453,  1917. 

125.  Hilgard,  E.  W.     Soils.     New  York,  190U. 

12(1  Hibbard,  R.  P.,  and  Harrington,  O.  E.  Depression  of  the  freezing-poinl 
in  triturated  plant  tissues  and  the  magnitude  of  this  depression  us  related 
to  soil  moisture.     Phys.  Res.,  Vol.  I,  No.  10,  1916. 

127.  Hoagland,  D.  F.     The  freezing  point  method  as  an  index  of  variations  in  the 

soil  solution  due  to  season  and  crop  growth.     U.  S.  Dept.  Agr..  Jour.  Agr. 
Res..  XII,  6,  Febr.  11,  1918. 

128.  Jones,  H.  C.     Physical  chemistry.     4th  ed. ,191s.     New  York. 

129.  King,  F.  H.    Investigations  in  soil  management.    V .  S.  Dept.  Agr.,  Bur.  of  Soils, 

Bull.  26,  1905. 
129a,  Livingston,  B.  E.,  Britton,  J.  C,  and  Reid,  F.  II.     Studies  on  the  properti< 
of  an  unproduetive  soil.     U.  S.  Dept.  Agr.,  Bur.  of  Soils,  Bull.  28,  1"" 

130.  :.     Review  of  Hans  Fittings'  Die  Wasserversorgnung  und  die  <  )smotisch< 

Druckverhaltnisse  der  wustenpflanzen.     Plant  World,  11.7.  1911. 

131.  McCool,  M.  M.,  and  Millar,  C.  E.     The  water  content  of  the  soil  and  the  .cm- 

position  and  concentration  of  the  soil  solution  as  indicated  by  the  freezing- 
point  lowerings  of  the  roots  and  tops  of  plants.     s,.il  Science,  vol 
Xo.  2,  1917. 


RESEARCH    METHODS  IX  STUDY  OF  FOREST  ENVIRONMENT.      207 

132.  McLaughlin,  W.  W.     Capillary  movement  of  soil  moisture.     U.  S.  Dept.  Agr 

Bull.  835,  1920.     Contribution  from  Bur.  Public  Roads. 

133.  Moore,  Barrington.     Osmotic  pressure  as  an  index  of  habitat.     Jour.  Forestry 

XV,  8,  Dec.  1917. 

134.  Nernst,  W.     Theoretical  chemistry.     7th  German  edition.     London,  1916. 

135.  Osborn,  H.  F.     The  origin  and  evolution  of  life.     New  York,  1918. 

136.  Patten,  II.  E.,  and  Gallagher,  F.  E.    Absorption  of  vapors  and  gases  by  soils. 

U.  S.  Dept.  Agr.,  Bur.  of  Soils,  Bull.  51,  1908. 

137.  Schull,  H.  A.     Measurement  of  the  surface  forces  in  soils.     Bot.  Gaz.,  vol 

pp.  1-31.  1916. 

138.  Schreiner.  ().,  and  Skinner,  J.  J.      Nitrogenous  soil  constituents  and  their 

bearing  on  soil  fertility.     U.  S.  Dept.  Agr.,  Bur.  Soils,  Bull.  87,  1912. 

139.  Shreve,  Forrest.     Rainfall  as  a  determinant  of  soil  moisture.     Plant  World, 

vol.  17.  Xo.  1,  1914. 

140.  Soils,  Bureau  of.     Soil  survey  field  book.     U.  S.  Dept.  Agr.,  1906. 

141.  Whereby,  E.  T.     Soil  acidity  and  a  field  method  for  its  measurement.     Ecology. 

I,  3.  L920. 

142.  Whitney,  M.  I).     Methods  of  the  mechanical  analysis  of  soils  and  of  the  de- 

termination of  the  amount  of  moisture  in  soils  in  the  field.     U.  S.  Dept. 
A.gr.,  Bur.  of  Soils,  Bull.  4,  1896. 

WIND    MOVEMENT. 

145.  Bates,  <'.  <i.     The  role  of  light  in  natural  and  artificial  reforestation.     Jour. 

Forestry.  XV,  2,  1917. 

146.  Humphreys,  W.  J.     Wind  velocity  and  elevation.     U.  S.  Weather  Bur.,  Mo. 

Weather  Rev..  XLIV,  I,  p.  14-17,  Jan.,  1916. 

147.  Marvin,  C.   F.     Anemometer  tests.     U.  S.  Weather  Bur.,  Mo.  Weather  Rev.. 

XXVI II.  2.  p.  58  63,   Feb.,  1900. 

148.  Sandstrom,  J.  W.     Working  up  wind  observations.     lT.  S.  Weather  Bur.,  Mo 

Weather  Rev.,  XLIII.  II.  p.  547-556,  Nov.,  1915. 

149.  U.  S.  Weather   Bureau.     Instru<  tions  for  the  installation  and  maintenance 

of  wind  measuring  and  recording  apparatus.     (Circ.  D,  Instrument  Div. 
U.  S.   Dept.  Agr.,  Weather  Bull.  530,  1914. 

150.  Weidman,  IF  II.     The  windfall  problem  in  the  Klamath  region,  Oregon.     Jour. 

Forestry.  XVIII.  S,  1920. 

EVAPORATION. 

151.  Bates,  < ' .  G.     A  new  evaporimeter  for  use  in  forest  studies.     U.  S.  Weather  Bur., 

Mo.  Weather  Rev..  May,  1919. 

152.  Bigelow,  F.  H.     A  manual  for  observers  in  climatology  and  evaporation.     IJ.  S. 

Dept.  Agr.,  Weather  Bull.  Xo.  409,  106  p.,  1909. 

153.  Briggs,  L.  J.,  and  Shantz.  H.  L.     The  water  requirements  of  plants.     U.  S. 

Dept.  Agr..  Bur.  Plant  Ind.,  Bulls.  284  and  285. 
154-  •         -  -•     Relative  water  requirements  of  plants.     U.   S.   Dept.   Agr., 

Jour.  Agr.  Res.,  Ill,  1,  1914. 

155.  Kadel,  B.  C.     Instructions  for  installation  and  operation  of  Class  A  evaporation 

stations.     U.  S.  Weather  Bur.     (Instrument  Div.,  Circ.  L.),  1915. 

156.  Kiesselbach,  T.  A.     Transpiration  as  a  factor  in  crop  production.     Xebr.,  Agr. 

Exp.  Station  Bull.  6,  1916. 

157.  Kimball,  H.  II.     Evaporation  observations  in  United  States.     U.  S.  Weather 

Bur.,  Mo.  Weather  Rev.,  XXXII,  12,  p.  559,  Dec,  1904. 
lo8.  Livingston,  B.  E.     A  rain  correcting  atmometer  for  ecological  instrumentation. 
Plant  World,  13,  p.  78-83,  1910. 


208        BULLETIN    1059,    U.    S.    DEPARTMENT    OF    AGRICULTURE. 

159.  Livingston-.  B.  E.     Operation   of   the  porous-cup   atmometer.     Plant  World, 

vol.   13,   p.  -112-113.   1910. 
160    m     Atmospheric  influence  on  evaporation  and  its  direct    measurement. 

U.  S.  Weather  Bur.,  Mo.  Weather  Rev.,  XLIII,  3,  p.  126-131,  March, 

1915. 

161.  }  and  Shreve,  E.  B.     Improvements  in  the  method  for  determining  the 

transpiring  power  of  plant  surfaces  by  hygrometric  paper.     Plant  World, 
Oct.,  1916. 

162.  Livingston,    Grace    J.     An    annotated    bibliography    of    evaporation.     Mo. 

Weather  Rev.,  vols.  XXXVI  and  XXXVII.  June,  1908,  to  Jan.'.  L909. 

163.  M  AnviN,  C.  F.     Methods  and  apparatus  for  observation  and  study  of  evaporation. 

1.  Methods,  U.  S.  Weather  Bur.,  Mo.  Weather  Rev..  April,  1909. 

2.  Instruments,  U.  S.  Weather  Bur.,  Mo.  Weather  Rev..  May,  1909. 

164.  Russell,  Thomas.     Piche  evaporimeter  (description  and  use).     U.S.  Weather 

Bur.,  Mo.  Weather  Rev.,  p.  253-255.     (Ed.  rev.  of  article  Mo.  Weather 
Rev.,  Sept.,  1888),  June,  1905. 

165.  Shreve,    J.    W.     An   improved    nonabsorbing   porous-cup    atmometer.     Plant 

World,  vol.  18.  Xo.  1,  p.  7-10,  Jan.,  1915. 
16G.  Shreve,  Forrest.     (See  Reference  16.) 

167.  Smith,  A.  W.     Our  present  knowledge  regarding  heat  of* evaporation  of  water. 

U.S.  Weather  Bur..  Mo.  Weather  Rev..  XXXV,  10.  p.  458  63,  Oct.,  L907. 

168.  Thom,  C.  C,  and  Holtz,  H.  F.     Factors  influencing  the  water  requirements  of 

plants.     Wash.  Agr.  Exp.  Station.  Bull.  L46,  1917. 

169.  Weaver,  J.  E.,  and  Thiel,  A.  F.     Ecological  studies  in  the  tension  zone  between 

prairie  and  woodland.     Univ.  oi-Xebr.  Botanical  Survey  .  Lincoln,  L917. 


ADDITIONAL  COPIES 

OF  THIS  PUBLICATION  MAY  BE  PROCURED  FROM 

THE  SUPERINTENDENT  OF  DOCUMENTS 

GOVERNMENT  PRINTING  OFFICE 

WASHINGTON,   D.   C. 

AT 

20  CENTS  PER  COPY 


^ 


: 


% 


' 


